Women are started on a lower daily dose of metoprolol than men irrespective of dose recommendations: A potential source of confounding by contraindication in pharmacoepidemiology.

ABSTRACT

PURPOSE: Current guidelines have no sex-specific dosage advice for metoprolol. To evaluate whether women and men are prescribed the same dose a cohort analysis was performed in the population-based Rotterdam Study (RS). Results were replicated in the Integrated Primary Care Information (IPCI) database of automated general practice data. METHODS: The mean daily starting doses of metoprolol in both sexes were compared with independent-samples t-tests and a linear regression analysis was used to adjust in the RS for co-variables, notably, cardiovascular comorbidity, migraine, age, SBP, DBP, BMI, socioeconomic status, use of other antihypertensive drugs, smoking, and alcohol. In the IPCI-database, adjustment was for age only. RESULTS: The mean daily starting dose was statistically significantly lower in women than in men in both the RS and IPCI database, with a mean difference of 4.8 mg (95%CI -7.8, -1.8) and 4.6 mg (95%CI -5.3,-4.0), respectively. Statistical significance remained after adjustment in both databases. CONCLUSIONS: Women received lower starting doses of metoprolol than men in two independent data collections despite non-sex specific cardiovascular guideline recommendations. This example of real-life pharmacotherapy can lead to a form of confounding by contraindication in pharmacoepidemiology.