Most of the bad side effects are things that people notice right away, such as changing your mental health for the worse instead of for the better. However, if it didn't help at least some people, the FDA would never have approved it.
In the long term, it can make existing heart problems worse. If it really helps you, though, it's probably worth it, assuming your heart is healthy. Just make sure you get regular checkups, so that any potential heart problems are detected right away.