A case of scleroderma-like (hardening of the skin) illness has been reported in a patient taking L-5-hydroxytryptophan and carbidopa (medication often used in the treatment of Parkinson's disease). An earlier case reported a similar interaction between carbidopa and 5-HTP. This interaction may be due to an alteration in tryptophan metabolism. More research is needed but it is advisable to avoid 5-hydroxytryptophan if you are currently taking carbidopa.

Individuals taking antidepressant drugs, including MAOIs (such as Nardil or Parnate) and selective serotonin re-uptake inhibitors (SSRIs) (such as: Prozac, Paxil, Zoloft) should consult with their physician before taking 5-HTP.