We get asked this question a lot. Here's a quick video to explain:
The main warning for supplements containing 5HTP is “do not consume if you’re taking antidepressant drugs.”
This is because antidepressant drugs also affect serotonin levels, but in a different way to 5HTP. Instead of increasing the creation of serotonin, antidepressant drugs like prozac are serotonin inhibitors. Which means they prevent the serotonin you already have from going away and it just hangs around.
So, taking both 5HTP and antidepressant drugs together would mean the body would create more serotonin and not be able to get rid of it which could lead to too much serotonin and serotonin syndrome.
That's why you should not take 5htp if you're on antidepressant drugs.