Note that creatine should be avoided if you're bipolar, according to a study or two and a lot of anecdotes.

Kidneys: https://examine.com/nutrition/does-creatine-cause-kidney-pro...

Overall, it's a very safe and moderately effective supplement, particularly if you don't eat a lot of red meat. Just make sure to drink enough water.

I wouldn't say I've been depressed (a psychiatrist might disagree, I've been dx'ed with it in the past), but daily 5mg creatine and 2k IU vitamin D have made a remarkable difference in mood/energy for me this year.