Unfortunately there has to be almost no side effects for almost all users, as there are no (as yet) medical benefits to male contraception.

In women, not being pregnant can prevent death for some of them, regulate painful periods, etc. - it is considered the risk of the myriad side effects is worth it because at least it does some good.

For men, who do not become pregnant, not being able to get someone else pregnant is not a medical benefit for the man.

And unfortunately hormonal modification does cause problems. Lots of them.