If you are being treated with any of the following medications, you should not use melatonin without first discussing it with your health care provider.

Antidepressant medications -- In an animal study, melatonin supplements reduced the antidepressant effects of desipramine and fluoxetine. More research is needed to determine whether these effects would occur in people. In addition, fluoxetine (a member of a class of drugs called selective serotonin reuptake inhibitors, or SSRIs) has led to measurable depletion of melatonin in people.

Antipsychotic medications -- A common side effect of antipsychotic medications used to treat schizophrenia is a condition called tardive dyskinesia, a movement disorder of the mouth characterized by a constant chewing motion and darting action of the tongue. In a study of 22 people with schizophrenia and tardive dyskinesia caused by antipsychotic medications, those who took melatonin supplements had significantly reduced mouth movements compared to those who did not take the supplements.

Benzodiazepines -- The combination of melatonin and triazolam (a benzodiazepine medication used to treat anxiety and sleep disorders) improved sleep quality in one study. In addition, a few reports have suggested that melatonin supplements may help individuals stop using long-term benzodiazepine therapy. (Benzodiazepines are highly addictive.)

Blood pressure medications -- Melatonin may reduce the effectiveness of blood pressure medications like methoxamine and clonidine. In addition, medications in a class called calcium channel blockers (such as nifedipine, verapamil, diltiazem, amlodipine, nimodipine, felodipine, nisoldipine, and bepridil) may decrease melatonin levels.

Use of beta-blockers (another class of high blood pressure medications that includes propranolol, acebutolol, atenolol, labetolol, metoprolol, pindolol, nadolol, sotalol, and timolol) may reduce melatonin production in the body.

Blood-thinning medications, anticoagulants -- Melatonin may increase the risk of bleeding from anticoagulant medications such as warfarin.

Interleukin-2 -- In one study of 80 cancer patients, use of melatonin in conjunction with interleukin-2 led to more tumor regression and better survival rates than treatment with interleukin-2 alone.

Nonsteroidal anti-inflammatory drugs (NSAIDs) -- NSAIDs such as ibuprofen may reduce the levels of melatonin in the blood.

Steroids and immunosuppressant medications -- People should not take melatonin with corticosteroids or other medications used to suppress the immune system because the supplement may cause them to be ineffective.

Tamoxifen -- Preliminary research suggests that the combination of tamoxifen (a chemotherapy drug) and melatonin may benefit certain patients with breast and other cancers. More research is needed to confirm these results.

Other Substances -- Caffeine, tobacco, and alcohol can all diminish levels of melatonin in the body while cocaine and amphetamines may increase melatonin production.