Why isn't there a male contraceptive pill?
The reason we don't have a male contraceptive pill isn't for lack of trying to develop one. There have been many studies and attempts to create a functional and safe male contraceptive pill that have not been successful. But why?
Biological differences between men and women
One of the main reasons we don't have one yet is the biological differences between men and women. For female contraceptive pills to work, you only need a small amount of hormones to trick the ovaries into thinking that the person taking the pill is already pregnant. When this happens, ovulation stops, meaning no eggs are produced, and the person taking the pill can't fall pregnant.
Men make sperm differently from how women make eggs, and there isn't a way to stop sperm from being made easily by introducing a hormone, as we do with the female contraceptive pill. The only way to do this would be with a much higher hormone dose that could lead to unmanageable side effects and possible complications. Moreover, men can still impregnate women for up to 3 months after they have stopped producing sperm, so this would mean men wouldn't be able to have unprotected sex without a risk of pregnancy for up to 3 months even when taking the pill.
Another possible reason we don't have a male contraceptive pill yet is that it is sometimes thought that as men do not have to carry the pregnancy, they're considered less likely to put up with the side effects a contraceptive pill might cause. However, new research has shown that only 26% of respondents' concerns were about the health implications that a male contraceptive pill could cause, which is a positive sign for people feeling comfortable taking it.
There have also been concerns about people's attitudes towards taking it and whether women would trust men to take it reliably. A recent survey showed that 42% of female respondents were concerned that men would forget to take their pill. So even if a pill was developed, would women trust men to take it over using the contraceptive pill themselves? Putting trust aside, the same survey showed that 49% of men would be willing to take the pill, and while we can't say for sure how reliable this would be, it's a positive trend and much closer to being in line with the 65% of women currently taking the female contraceptive pill.
In summary, we don't have a contraceptive pill for men yet because we haven't been able to develop one that's both functional and entirely safe for use. There are also underlying issues regarding trust in taking the pill reliably, health concerns, and (changing) social attitudes towards who should be responsible for contraception. There is work being done to solve these issues, but it may be some time yet until a contraceptive pill for men is ready.