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metabolism of melatonin in the liver, such as valproic acid or methoxypsoralen, could
affect serum levels of melatonin.
They further caution that melatonin is likely to modulate the effects of other
substances, which requires careful consideration of appropriate dosages if any other
drugs or herbs are being used, and may require that Mischoulon and Rosenbaum’s
lower melatonin dosages be increased.
Likewise, generally, the Natural Standard cautions that melatonin may increase
drowsiness caused by other drugs, such as zolpidem (Ambien), benzodiazepines such as
lorazepam (Ativan) or diazepam (Valium), barbiturates such as phenobarbital, narcotics
such as codeine, some antidepressants (unspecified) and alcohol.
The Natural Standard cautions generally that since melatonin is broken down
(metabolized) by the liver enzymes, drugs that alter the activity of these enzymes may
increase or decrease effects of melatonin supplements.
“Based on preliminary evidence,” the Natural Standard cautions that melatonin should
be avoided in people taking the blood thinning medication warfarin (Coumadin) and
possibly by people using other blood thinners (anticoagulants) such as aspirin or heparin
if problems become apparent.
Since melatonin may cause adverse changes in blood pressure and cholesterol levels,
caution is advised to persons on blood pressure or cholesterol medication and persons
with or at risk of cardiovascular disease. Abnormal heart rhythms have been reported.
It is not clear if or how caffeine affects melatonin supplementation. Caffeine may raise
melatonin levels, but its stimulative effects may also alter wake-sleep rhythms.
Alcohol consumption seems to alter melatonin secretion at night.
According to the Natural Standard: "Preliminary reports suggest that melatonin may aid
in reversing ... tardive dyskinesia associated with [use of] haloperidol... [and] may
increase the effects of isoniazid against [tuberculosis].”
On the other hand, melatonin may increase the adverse effects of methamphetamine
on the nervous system.