Transfer of Dicloxacillin into Human Milk.

Background: Dicloxacillin is a beta-lactam antibiotic that is commonly used in the treatment of lactational mastitis in breastfeeding women. Although penicillins have long been considered safe for breastfeeding mothers and their infants, there is almost no data on the transfer of dicloxacillin into human breast milk despite the fact that it is commonly used for mastitis. Case Report: This study determined the drug concentration-time profile of dicloxacillin in milk samples collected from three lactating mothers consuming 500 mg dicloxacillin taken every 6 hours for treatment of mastitis. Milk levels were measured using liquid chromatography mass spectrometry. The maximum concentration of dicloxacillin in milk was 67.6 ng/mL. The relative infant dose (RID) was calculated to be 0.03%. This value is well below the theoretical level of concern of 10%. Discussion: The limited transfer of dicloxacillin into human milk is probably explained by the high plasma protein binding of dicloxacillin and its subsequent poor penetration into human milk. Conclusion: In this case series, the level of dicloxacillin in milk was found to be very low, and the RID to be only 0.03% of the maternal dose. Although the levels detected were low, dicloxacillin does transfer into breast milk. Caution should be exercised in infants with hypersensitivity to penicillins.

Acquired pyroglutamic acidosis due to long-term dicloxacillin and paracetamol use.

An 85-year-old man with a background of transfusion-dependent chronic myelomonocytic leukaemia and chronic kidney disease stage III presented with symptomatic anaemia, acute kidney injury, sepsis and high anion gap metabolic acidosis (HAGMA). Initial treatment with intravenous antibiotics and blood transfusion was complicated by transfusion-associated circulatory overload, necessitating diuresis and non-invasive ventilation. Despite gradual clinical improvement, the patient's HAGMA persisted, and no cause was identified on urine testing or renal ultrasound. As the patient was on long-term dicloxacillin for infective endocarditis prophylaxis and regular paracetamol, pyroglutamic acidosis (PGA) (5-oxoproline acidosis) was considered. This was later confirmed with elevated serum levels, and the HAGMA resolved following cessation of these medications. Although considered an uncommon cause of HAGMA, PGA is likely also under-recognised, and to our knowledge, this may be the second reported case in the context of dicloxacillin.

Exploring anti-MRSA activity of chitosan-coated liposomal dicloxacillin.

One of the greatest disturbing global health problems is antibiotic-resistant bacterial infections, which have rendered numerous currently used antibiotics ineffective. Thus, the feasibility of chitosan-coated deformable liposomes (C-Lips) containing dicloxacillin (DLX) were evaluated for their efficacy against methicillin-resistant Staphylococcus aureus (MRSA) strains, which are resistant to beta lactam antibiotics. DLX-loaded liposomes (DLX-Lip) were prepared by a lipid film hydration method and then chitosan (CS) coated (C-DLX-Lip) by the electrostatic deposition method. Both DLX-Lips and C-DLX-Lips showed a particle size distribution with a nano-range and a narrow polydispersity index (PDI). After CS coating, the zeta potential was shifted from negative to positive value. The DLX entrapment efficiency (EE) and drug loading (DL) were 62% and 5.6% for C-DLX-Lips compared to 38% and 3.1% for DLX-Lip, respectively. The in vitro release profile of C-DLX-Lips possessed a slow release behavior. Moreover, the DLX-Lips and C-DLX-Lips demonstrated an enhanced anti-MRSA activity. These results revealed that DLX-Lips and C-DLX-Lips may serve as promising carriers for DLX to increase the efficacy against MRSA, which offers considerably clinical value for long-term use of DLX.

Use of Dicloxacillin and Risk of Pregnancy among Users of Oral Contraceptives.

The antibiotic dicloxacillin has been shown to induce drug-metabolizing CYP enzymes to a clinically relevant extent. In this study, we investigated whether the use of dicloxacillin confers an increased risk of unwanted pregnancy among oral contraceptive users. The study population comprised Danish women falling pregnant (1997-2015) during oral contraceptive use, defined as having filled a prescription for an oral contraceptive within 120 days both before and after the estimated date of conception. Data were analysed using a case-crossover approach. For each woman, we assessed the use of dicloxacillin preceding the date of conception and during 10 previous control periods and estimated the odds ratio for such unintended pregnancies associated with the use of dicloxacillin. Among 364 women using dicloxacillin prior to conception, 40 (11%) were exposed to dicloxacillin at the time of conception, yielding an odds ratio (OR) associating use of dicloxacillin to unintended pregnancy of 1.18 (95% CI 0.84-1.65). Supplementary and sensitivity analyses generally returned similar estimates, except for a slightly increased risk among users of progestogen-only oral contraceptives (OR 1.83, 95% CI 0.63-5.34). Analysis of other antibiotics as negative controls yielded results close to unity (ORs ranging from 0.83 to 1.13). In conclusion, our study found no evidence for an increased risk of oral contraceptive failure when using dicloxacillin. However, acknowledging study limitations, we suggest the use of supplementary barrier methods during treatment with dicloxacillin, until our findings are confirmed in further studies.

Microdialysis as a tool to determine the local tissue concentration of dicloxacillin in man.

AIMS: The most common pathogen to cause postoperative infections in Denmark is Staphylococcus aureus. Despite using prophylactic antibiotics, infections are still seen. Whether the tissue concentration is above the minimal inhibitory concentration (MIC) for the pathogen is unknown. Thus, the concentration of dicloxacillin in muscle and adipose tissue was measured after intravenous administration, in healthy men. METHODS: MIC for dicloxacillin against S. aureus was determined using the broth macrodilution method. A microdialysis (MD) catheter was placed in the subcutaneous tissue of the abdomen and in the lateral vastus muscle of the thigh of six healthy male volunteers. They were given 2 g dicloxacillin intravenously. Samples from blood and MD fluid were collected. The unbound dicloxacillin was isolated from plasma. Samples were analysed with high performance liquid chromatography (HPLC). RESULTS: The maximum concentration was reached in muscle tissue after 0.5 h and in adipose tissue after 0.8 h. AUC0-6h for the dicloxacillin concentration in adipose tissue was significantly lower when compared to the unbound dicloxacillin concentration in plasma. The dicloxacillin concentration was above the MIC for sensitive S. aureus for a minimum of 2.3 h and a median of 4.1 h in muscle tissue and a minimum of 1.8 h and a median of 3.2 h in adipose tissue. CONCLUSIONS: The unbound dicloxacillin concentration in adipose and muscle tissue remained above the MIC for sensitive S. aureus, for a period sufficient for many orthopaedic procedures. Whether this is true in patients with compromised circulation remains to be investigated.

Determining optimal dosing regimen of oral administration of dicloxacillin using Monte Carlo simulation.

BACKGROUND: Dicloxacillin, a semisynthetic isoxazolyl penicillin, exhibits antimicrobial activity against a wide variety of Gram-positive bacteria, as well as stability against penicillinases and low level of toxicity. The objective of this study was to obtain optimal dosing regimen of oral administration of dicloxacillin by analyzing the pharmacokinetic (PK) index in healthy volunteers and in vitro antibacterial activity by using Monte Carlo simulation. MATERIALS AND METHODS: A total of 867 clinical isolates from community-onset infections were collected from 31 secondary hospitals in People's Republic of China. The minimum inhibitory concentration (MIC) values of dicloxacillin were determined by the agar dilution method. Based on the MICs and the PK parameters of different dosage regimens, Monte Carlo simulation was performed to simulate the PK/pharmacodynamic indices of 250 mg once-daily (qd), 500 mg qd, 1,000 mg qd, 2,000 mg qd, 250 mg every 6 hours (q6h), and 500 mg q6h, respectively. The probability of target attainment was estimated at each MIC value, and the cumulative fraction of response (CFR) was calculated to evaluate the efficacy of these regimens. RESULTS: Dicloxacillin showed poor antibacterial activity against Haemophilus influenzae, Moraxella catarrhalis, and Streptococcus pneumoniae. Resistance to dicloxacillin was observed in 7.5% of coagulase-negative Staphylococcus (CNS) isolates and 9.2% of other Streptococcus isolates, whereas 1.5% of methicillin-sensitive Staphylococcus aureus (MSSA) was resistant to dicloxacillin. Multiple-dose regimens could obtain higher CFR than single-dose regimens against H. influenza and S. pneumoniae. However, all dosing regimens against MSSA achieved CFR ≥$90%. Meanwhile, dosing regimen of 2,000 mg qd, 250 mg q6h, and 500 mg q6h could achieve >90% of CFR for CNS. For other Streptococcus isolates, multiple-dose regimens achieved CFR ≥90%. CONCLUSION: Dicloxacillin has a significant antibacterial activity against MSSA, CNS, and other Streptococcus isolates. The simulation results suggest that dicloxacillin 250 mg q6h and 500 mg q6h dosing regimens may be recommended for clinical applications, especially for community-onset infections.

Combination of thioridazine and dicloxacillin as a possible treatment strategy of staphylococci.

We have previously shown that the phenothiazine, thioridazine, acts in synergy with the beta-lactam antibiotic, dicloxacillin, to kill methicillin-resistant Staphylococcus aureus. In this study, we investigated whether synergy by combining these two drugs could also be observed in vancomycin intermediate susceptible S. aureus (VISA) and methicillin-resistant Staphylococcus epidermidis (MRSE). Synergy was observed in three of four tested VISA strains, suggesting that the thickening of cell wall does not interfere with the effects of thioridazine. In S. epidermidis, no synergy was observed in all tested strains, suggesting that synergy by combining thioridazine and dicloxacillin is isolated to S. aureus species.

Safety and pharmacokinetics of dicloxacillin in healthy Chinese volunteers following single and multiple oral doses.

BACKGROUND: Dicloxacillin, a semisynthetic isoxazolyl penicillin antibiotic, has antimicrobial activity against a wide variety of gram-positive bacteria including Staphylococcus aureus, Streptococcus pyogenes, Streptococcus pneumonia, Streptococcus epidermidis, Streptococcus viridans, Streptococcus agalactiae, and Neisseria meningitidis. The objective of this study was to evaluate the safety and pharmacokinetic profile of dicloxacillin after single and multiple oral dose in healthy Chinese volunteers. METHODS: A single-center, open-label, randomized, two-phase study was conducted in 16 subjects. In the single-dose phase, subjects were randomly assigned to receive single doses of 0.25, 0.5, 1.0, and 2.0 g of dicloxacillin sodium capsule in a 4-way crossover design with a 5-day washout period between administrations. In the multiple-dose phase, subjects were assigned to receive 0.25 or 0.5 g every 6 hours for 3 days in a 2-way crossover design. Plasma and urine pharmacokinetic samples were assayed by a validated high-performance liquid chromatography-tandem mass spectrometry method. Pharmacokinetic parameters were calculated and analyzed statistically. Safety assessments were conducted throughout the study. RESULTS: Following a single oral dose of 0.25-2.0 g dicloxacillin sodium, the maximum plasma drug concentration (Cmax) and the corresponding values for the area under the concentration- time curve from 0 to 10 hours (AUC0-10 h) increased in a dose-proportional manner. The mean elimination half-life (t1/2) was in the range of 1.38-1.71 hours. Dicloxacillin was excreted in its unchanged form via the kidney, with no tendency of accumulation, and varied from 38.65% to 50.10%. No appreciable accumulation of drug occurred with multiple oral doses of dicloxacillin. No serious adverse events were reported. Adverse events were generally mild. CONCLUSION: Dicloxacillin was safe and well tolerated in the volunteers and displayed linear increases in the Cmax and AUC0-10 h values.

Combination therapy with thioridazine and dicloxacillin combats meticillin-resistant Staphylococcus aureus infection in Caenorhabditis elegans.

The shortage of drugs active against meticillin-resistant Staphylococcus aureus (MRSA) is a growing clinical problem. In vitro studies indicate that the phenothiazine thioridazine (TZ) might enhance the activity of the ß-lactam antibiotic dicloxacillin (DCX) to a level where MRSA is killed, but experiments in simple animal models have not been performed. In the present study, we introduced Caenorhabditis elegans infected by S. aureus as an in vivo model to test the effect of TZ as a helper drug in combination with DCX. Because TZ is an anthelmintic, initial experiments were carried out to define the thresholds of toxicity, determined by larval development, and induction of stress-response markers. No measurable effects were seen at concentrations of less than 64 mg TZ l(-1). Seven different MRSA strains were tested for pathogenicity against C. elegans, and the most virulent strain (ATCC 33591) was selected for further analyses. In a final experiment, full-grown C. elegans were exposed to the test strain for 3 days and subsequently treated with 8 mg DCX l(-1) and 8 mg TZ l(-1) for 2 days. This resulted in a 14-fold reduction in the intestinal MRSA load as compared with untreated controls. Each drug alone resulted in a two- to threefold reduction in MRSA load. In conclusion, C. elegans can be used as a simple model to test synergy between DCX and TZ against MRSA. The previously demonstrated in vitro synergy can be reproduced in vivo.

Intravenous antibiotics given for 2 weeks do not eradicate persistent Staphylococcus aureus clones in cystic fibrosis patients.

Staphylococcus aureus is the most commonly isolated pathogen in respiratory tract secretions from young patients with cystic fibrosis (CF), and several treatment strategies are used to control the infection. However, it is not known whether intensified treatment with antimicrobial agents causes eradication of S. aureus clones. We retrospectively determined the impact of intravenous (IV) antimicrobial agents on the suppression and eradication of S. aureus clones. One thousand and sixty-one S. aureus isolates cultured from 2526 samples from 130 CF patients during a 2-year study period were subjected to spa typing. Intervals between positive samples and the occurrence of clone replacements were calculated in relation to courses of IV antimicrobial agents. Of 65 patients chronically infected with S. aureus, 37 received 139 courses of IV antimicrobial agents with activity against S. aureus (mean duration, 15 days; range, 6-31 days). Administration of IV antibiotics increased the time to the next sample with growth of S. aureus: the mean interval between two positive samples was 68 days if IV treatment had been administered, in contrast to 49 days if no IV treatment had been administered (p 0.003). When S. aureus recurred in sputum after IV treatment, the isolate belonged to a different clone in 33 of 114 (29%) intervals, in comparison with 68 of 232 (29%) intervals where IV treatment had not been prescribed (OR 0.98, 95% CI 0.60-1.61). In conclusion, we show that 2 weeks of IV antimicrobial treatment can significantly suppress chronic staphylococcal infection in CF, but is not associated with the eradication of persistent bacterial clones.

[Bacterial pyomyositis in a 12-year-old girl without fever]. / Pyomyositis uden feber hos 12-årig pige.

Bacterial pyomyositis is generally found in tropical countries. This case report presents pyomyositis in a 12-year-old girl who was admitted without fever to the paediatric department. The only symptom was pain in the left hip. Staphylococcus aureus was cultured from the blood on day 4. Magnetic resonance imaging (MRI) revealed infection in the left m. ileopsoas. Previous ultrasound, computerised tomography, x-ray and bone-scintigraphy were normal. After 11 days of intravenous antibiotic therapy and clinical remission, secondary bone affection was detected by a new MRI. Long-term antibiotic treatment is required in such cases because of the risk of secondary bone affection. This patient was treated for 11 days with intravenous antibiotic therapy and for the subsequent three months with tablets.

Intracellular activity of antibiotics against Staphylococcus aureus in a mouse peritonitis model.

Antibiotic treatment of Staphylococcus aureus infections is often problematic due to the slow response to therapy and the high frequency of infection recurrence. The intracellular persistence of staphylococci has been recognized and could offer a good explanation for these treatment difficulties. Knowledge of the interplay between intracellular antibiotic activity and the overall outcome of infection is therefore important. Several intracellular in vitro models have been developed, but few experimental animal models have been published. The mouse peritonitis/sepsis model was used as the basic in vivo model exploring a quantitative ex vivo extra- and intracellular differentiation assay. The intracellular presence of S. aureus was documented by electron microscopy. Five antibiotics, dicloxacillin, cefuroxime, gentamicin, azithromycin, and rifampin (rifampicin), were tested in the new in vivo model; and the model was able to distinguish between their extra- and intracellular effects. The intracellular effects of the five antibiotics could be ranked as follows as the mean change in the log(10) number of CFU/ml (Delta log(10) CFU/ml) between treated and untreated mice after 4 h of treatment: dicloxacillin (3.70 Delta log(10) CFU/ml) > cefuroxime (3.56 Delta log(10) CFU/ml) > rifampin (1.86 Delta log(10) CFU/ml) > gentamicin (0.61 Delta log(10) CFU/ml) > azithromycin (0.21 Delta log(10) CFU/ml). We could also show that the important factors during testing of intracellular activity in vivo are the size, number, and frequency of doses; the time of exposure; and the timing between the start of infection and treatment. A poor correlation between the intracellular accumulation of the antibiotics and the actual intracellular effect was found. This stresses the importance of performing experimental studies, like those with the new in vivo model described here, to measure actual intracellular activity instead of making predictions based on cellular pharmacokinetic and MICs.

Prescripción de medicamentos en una clínica odontológica de una universidad mexicana / No disponible

Objetivo: Evaluar la prescripción de medicamentos en una clínica odontológica de una universidad mexicana. Método: Mediante un estudio observacional y descriptivo, se analizaron 698 prescripciones odontológicas en 14 servicios clínicos que conforman la clínica en estudio, enfocados a la conservación y restauración de la salud bucal en enero-junio 2005. Se registraron criterios como: medicamento prescrito, indicación, dosis, intervalo de dosificación, individualización de la terapia, duración de tratamiento y presencia de interacciones farmacológicas potenciales. Para determinar la inadecuación en los criterios de prescripción se comparó la información obtenida en recetas y expedientes clínicos, con la de la literatura especializada. Resultados: Los medicamentos más prescritos fueron paracetamol, naproxeno, ampicilina y dicloxacilina en 43,26, 15,38, 7,45 y 7,02%. La indicación, dosis e intervalo de dosificación fueron los criterios con mayor inadecuación en la prescripción. Las principales interacciones potenciales fueron entre los antiinflamatorios no esteroideos con el captopril y la amoxicilina. Conclusiones: Con lo anterior, se determinó que el 37,25% de las prescripciones fueron inadecuadas. A través de este estudio se establecieron estrategias que permitirán en un futuro tener una política de uso racional de los medicamentos empleados

Objective: To assess the drug prescription service in a dental clinic of a Mexican university hospital. Method: An observational, descriptive study was carried out which analysed 698 drugs prescribed for dental problems in 14 dental health care departments in our clinic between the period of January-June 2005. The following criteria were established: prescribed drug, indication, dosage, dosage interval, individualised treatment, treatment duration and potential drug interactions. Information taken from prescriptions and clinical records was compared with information from literature on the subject in order to determine the adequacy of prescription criteria. Results: The most frequently prescribed drugs were paracetamol, naproxen, ampicillin and dicloxacillin (43.26, 15.38, 7.45 and 7.02%). The prescription criteria which showed the least adequacy were as follows: indication, dosage and dosage interval. The main potential drug interactions occurred between non-steroidal anti-inflammatory drugs and captopril/amoxicillin. Conclusions: Taking the above into consideration, it was determined that 37.25% of prescriptions were inadequate. This study has helped to establish strategies which will facilitate the appropriate use of drugs in the future

Influence of sub-inhibitory concentrations of antimicrobial agents on biofilm formation in indwelling medical devices.

Biofilms of Staphylococcus epidermidis and Candida spp. are two of the most frequent factors of infections associated with the use of indwelling medical devices. Several strategies have been proposed and/or developed to prevent infection. The aim of this study was to compare the effect of sub-inhibitory concentrations of anti-microbial agents on biofilm formation. Biofilms of three strains of S. epidermidis and two of both Candida albicans and Candida dubliniensis were formed in the presence of three antibiotics and two antifungal agents respectively. Based on the control samples, the percentage of biofilm formation inhibition by the different agents was determined and compared. The results showed that the influence of the antibacterial and antifungal agents tested is strain dependent, with the effect of the different agents also varying among strains, even though they have the same mechanism of action.

Local gentamicin reduces sternal wound infections after cardiac surgery: a randomized controlled trial.

BACKGROUND: Sternal wound infections remain a major cause of morbidity after cardiac surgery. Vancomycin is often the only effective antibiotic available for their treatment but its use for routine prophylaxis is inadvisable for ecological reasons. Local application of gentamicin produces high antibiotic concentrations in the wound. We aimed to determine whether this treatment could have an additive effect on the incidence of sternal wound infections when combined with routine prophylaxis. METHODS: Two thousand cardiac surgery patients were randomized to routine prophylaxis with intravenous isoxazolyl-penicillin alone (control group) or to this prophylaxis combined with application of collagen-gentamicin (260 mg gentamicin) sponges within the sternotomy before wound closure. Endpoint was any sternal wound infection within 2 months postoperatively. Evaluations were double-blind and made on an intention-to-treat basis. RESULTS: Evaluation was possible in 967 and 983 patients in the control and treatment groups, respectively. The incidence of sternal wound infection was 4.3% in the treatment group and 9.0% in the control group (relative risk 0.47; 95% confidence interval 0.33-0.68; p < 0.001). Early reoperation for bleeding was more common in the treatment group (4.0% vs 2.3%, p = 0.03). No difference in postoperative renal function was noted. CONCLUSIONS: Local collagen-gentamicin reduced the risk for postoperative sternal wound infections. Further studies are warranted to confirm these results, particularly with regard to deep infections.

[Long-term antibiotic suppressive therapy for an infected thoracic aorta graft]. / Langtids suppresjonsbehandling med antibiotika av infisert torakal aortaprotese.

BACKGROUND: Infections of arterial vascular grafts are among the most dreaded complications in vascular surgery. Infection in a thoracic aorta graft poses particular challenges. Depending on the local anatomy, extraanatomic bypass, otherwise the cornerstone in the management of infected vascular grafts, is usually impossible. MATERIAL AND METHODS: We present a case report on a patient with an infected thoracic aorta graft and discuss the choice of antibiotics for long-term suppressive therapy. RESULTS: In the course of the 52 months since the insertion of the aortic graft, the patient experienced eight serious episodes of staphylococcal septicaemia, with Staphylococcus aureus in blood culture on each occasion. A combination therapy of three orally administered anti-staphylococcal antibiotics has kept him free from recurrent septic episodes over the last 25 months, with a good quality of life and without signs of systemic infection. INTERPRETATION: Life-long antibiotic suppressive therapy for infected thoracic aorta graft offers the prospect of long-term survival with a good quality of life, even when there are recurrent serious septic complications. The choice of antibiotics should take into account the feasibility of the proposed treatment; parenteral antibiotics are not a realistic option in the long run. The antibiotics should be well absorbed after oral administration, have a high intrinsic activity against the offending pathogen, and be given at intervals leading to inhibitory blood concentrations throughout most of the day. In our patient, a triple combination therapy was necessary.

Antibiotic concentrations in serum and wound fluid after local gentamicin or intravenous dicloxacillin prophylaxis in cardiac surgery.

One important aim of antibiotic prophylaxis in cardiac surgery is preventing mediastinitis and thus it would appear to be relevant to study the antibiotic concentrations in pericardial/mediastinal fluid. Local administration of gentamicin in the wound before sternal closure is a novel way of antibiotic prophylaxis and could be effective against bacteria resistant to intravenous antibiotics. This study measured dicloxacillin concentrations in 101 patients in serum and wound fluid following intravenous administration of dicloxacillin. Similarly, concentrations of gentamicin in serum and wound fluid were determined in 30 patients after administration of 260 mg gentamicin in the wound at sternal closure. Median dicloxacillin concentrations in serum and wound fluid at sternal closure were 59.4 and 55.35 mg/l, respectively. Gentamicin levels in the wound were very high (median 304 mg/l), whereas serum concentrations were low (peak median 2.05 mg/l). Dicloxacillin, 1 g given intravenously, according to the clinical protocol, resulted in levels in serum and wound fluid at sternal closure likely to prevent Staphylococcus aureus infections. Locally administered gentamicin resulted in high local concentrations, potentially effective against agents normally considered resistant.

Staphylococcal scalded skin syndrome complicating acute generalized pustular psoriasis.

A 60-year-old woman with psoriasis vulgaris treated with oral cyclosporin and acitretin developed an acute generalized pustular eruption with erythema and associated fever consistent with acute generalized pustular psoriasis. She was admitted to hospital and, despite intravenous fluid replacement, developed acute renal failure. In addition, she developed staphylococcal septicaemia. After transfer to the intensive care unit because of deteriorating renal function, a sudden onset of widespread flaccid blistering (Nikolsky sign positive) and superficial erosions was noted. Histology of a biopsied blister revealed subcorneal splitting of the epidermis consistent with staphylococcal scalded skin syndrome. The patient was treated with intravenous dicloxacillin and the blistering gradually improved over 10 days.

Milk depletion of dicloxacillin residues in cows and sheep following intramammary administration.

The excretion rate of dicloxacillin from milk was studied after intramammary administration of a suspension of the drug active in vegetable oil. Eight cows and eight sheep, four of each group in low and four in high milk production, were dosed with 200 mg dicloxacillin/quarter in cows and 100 mg dicloxacillin/quarter in sheep, three times at 12 h intervals. The dicloxacillin concentrations in milk were quantified by high performance liquid chromatography (HPLC). In cows, time until dicloxacillin was undetectable was 48 h and no difference was observed between the groups. In sheep, dicloxacillin was undetectable 72 h and 84 h after the treatment in low and in high milk production groups, respectively. The implications of several factors affecting the possible milk withdrawal period were studied. The results indicated that the pharmaceutical vehicle and the coefficient of lipid solubility exerted major effects on depletion time.