Strangely, oral contraceptives may alter the aroma of females who take them. That's according to an animal study published in 2010 in the scientific journal, Proceedings of the Royal Society B. Researchers at Duke Lemur Center studied the effect of medroxyprogesterone acetate, usually known by its brand name of Depo-Provera, on 12 adult female ring-tailed lemurs, and found that that the contraceptive altered the chemical composition of secretions from the lemurs' genital areas, making their scent less attractive to male lemurs [source: Bates].
Does the same hold true for humans? People are known to send and receive olfactory cues about hormonal status, and the Duke researchers said more study would need to be done to see whether humans can detect whether someone is on birth control through scent.
Meanwhile a 2008 study showed that being on birth control pills can affect a woman's taste in men. Women were asked to smell T-shirts worn by men and pick the ones they were most attracted to. They overwhelmingly picked ones for men whose immune systems were different from theirs. From an evolutionary point of view, this is a good thing as it ensures a more diverse gene pool. However, when the women were on the birth control pill, they tended to pick men with similar immune systems. Could that mean that they might end up with the wrong man? [source: Nalls]