Drugs that affect hormones have the potential to cause all kinds of screwy side effects in the body—but fortunately, there's not much evidence that medications like birth control or antidepressants will slow you down on the track or impede your progress in the gym. In fact, some research shows that they can actually improve athletic performance.
Take The Pill, for example (along with other forms of hormonal contraception, like the patch, the ring, and the IUD): "Birth control typically consists of estrogen and/or progrestrone," says Corey Hunter, M.D., "And it has been suggested there may be a link between estrogen and endurance." (In a 1998 study, post-menopausal women being treated with estrogen were found to have enhanced vascular endothelial function, thus improving their exercise endurance.) "More to the point, synthetic forms of estrogen and progesterone have been found to improve cardiac output and enhanced performance," Hunter adds.
One place women may see their fitness suffer, however, is in strength gains. A 2009 Texas A&M study found that subjects who took certain types of birth control containing synthetic progesterone did not gain as much lean muscle over the course of a 10-week weight-lifting plan. The study authors say that while the difference may be significant for professional body builders and Olympians, it likely wouldn't be too noticeable, or make much of a difference, to non-competitive athletes.
Antidepressants, on the other hand, work by blocking cells' reabsorption of serotonin and/or norepinephrine, making it more available to synapses in the brain. Exercise has also been shown to increase levels of these hormones, says Hunter, although researchers aren't exactly sure what the connection is.
What they do know is that different antidepressants work for different people: For many people, mood-boosting medication can increase a person's energy levels and desire to exercise. If you find that yours is doing just the opposite—zapping your drive, disrupting your sleep, or making your legs feel heavy when you run—talk to your doctor about the pros and cons of trying another one. Common side effects like drowsiness, dry-mouth, and weight gain could also potentially impact your workouts, but these can often be managed by getting adequate rest, staying hydrated, and otherwise maintaining a healthy lifestyle.
Bottom line: In most cases, antidepressants and birth control can help improve your overall fitness levels by reducing depressive symptoms and PMS, respectively. But if you suspect any kind of performance-related side effect, says Hunter, consult with your doctor about other potential treatment options.