I was given 500 mg by a naturopathic doctor. i am on40 mg of prozac (fluoxetine). are these two safe together. how risky is it for serotonin syndrome?

500 mg of what?: You neglected to tell us 500 mg of what so it's impossible to know, but I'm guessing 500 mg of L-tryptophan. While tryptophan and Prozac (fluoxetine) can theoretically cause serotonin syndrome, in a research study 2000 mg of tryptophan and 20 mg of Prozac (fluoxetine) was safe and showed better outcomes than Prozac (fluoxetine) alone: http://tinyurl.com/h7z53le 500 mg of Tryptophan is unlikely to cause this but watch for symptoms.