5-HTP should NOT EVER be combined with drugs that affect serotonin (such as SSRIs) due to a supposed (but not scientifically established) risk of suffering from serotonin syndrome - which in severe cases can be fatal.
The Americans have been prescribing 5-HTP for many years but nobody really has a full list of drug interactions for 5-HTP, so nobody knows for sure what you can and can't take it with.
However 5-HTP does seem to be effective when used alongside inositol. If you are using 5-HTP and a high dose of inositol, you may not need to use antidepressants.