Oral contraceptives do not affect gains in muscle size, strength, or power

Study Design: This analysis combined results from 8 studies involving 325 women (ages 18-40) who did resistance training for about 12 weeks. Researchers compared women taking birth control pills to those with natural menstrual cycles, measuring changes in muscle size, strength, and power.

Main Findings: Birth control pills had no effect on training results. Women on the pill gained the same muscle size (effect size: 0.01, p=0.90), strength (effect size: 0.10, p=0.20), and power (effect size: -0.04, p=0.80) as women not using hormonal contraception. All effect sizes were near zero with statistical tests showing no significant differences. The analysis of power was very limited due to small number of outcomes measured by only 3 studies.

The limitation is that studies were relatively short (8-16 weeks) and only examined birth control pills, not other contraceptive types. However, this is the strongest evidence available on this question, using meta-analysis to combine multiple studies.