Melatonin Drug Interactions: Sedatives, Warfarin, and More
Melatonin is widely used as a sleep aid and is generally considered safe at the doses sold over the counter, but it is not pharmacologically inert. It has real effects on the central nervous system and is metabolized by liver enzymes, both of which create opportunities for interactions with medications.
Additive sedation with CNS depressants
Melatonin promotes sleep by acting on MT1 and MT2 receptors in the brain, suppressing the alerting signal from the suprachiasmatic nucleus. This CNS-depressant effect adds to that of other substances that also depress central nervous system activity — benzodiazepines (lorazepam, diazepam, clonazepam), non-benzodiazepine sleep aids (zolpidem, eszopiclone), sedating antihistamines, opioids, and alcohol.
Combining melatonin with these substances may produce more sedation, impaired coordination, or cognitive blunting than either alone. This is a pharmacodynamic interaction — each substance independently suppresses CNS function and the effects add. It is not a metabolic interaction where one drug changes the blood level of another.
Metabolic interactions via CYP enzymes
The most significant metabolic interaction involves fluvoxamine (Luvox), an SSRI used for OCD and depression. Melatonin is primarily metabolized by CYP1A2 and CYP2C19 in the liver. Fluvoxamine is one of the most potent inhibitors of CYP1A2 available — when it blocks this enzyme, melatonin is not cleared from the body normally and plasma melatonin levels can increase by approximately 17-fold, per pharmacokinetic studies cited in the NIH ODS melatonin fact sheet. Even a low dose of supplemental melatonin becomes effectively a very high dose when fluvoxamine impairs its clearance. Most other SSRIs — sertraline, escitalopram, paroxetine, fluoxetine — do not have this effect because they inhibit different CYP enzymes. Fluvoxamine is specific because of its strong CYP1A2 inhibition.
Warfarin is a less well-characterized interaction. Some case reports have described INR changes — both increases and decreases — in patients on warfarin who added melatonin, but the evidence is limited to case reports and small studies. The mechanism is uncertain; melatonin may have some influence on CYP2C9, which metabolizes warfarin. The NIH ODS fact sheet notes melatonin's possible effect on anticoagulant and antiplatelet drugs, and patients on warfarin who begin melatonin may want to monitor INR more closely in the period after starting.
A few studies have also found that melatonin may blunt the blood-pressure-lowering effect of nifedipine, a calcium channel blocker. The mechanism is not fully established, but melatonin has some vasoconstrictive properties via melatonin receptor action. The effect appears modest and has not been consistently replicated. Additionally, melatonin has immunomodulatory effects — it can stimulate immune cell activity and cytokine production — which is theoretically counterproductive for people on immunosuppressant medications such as cyclosporine or tacrolimus after organ transplant. The NIH ODS fact sheet flags both of these as interaction categories.
Source: Melatonin — Health Professional Fact Sheet (NIH Office of Dietary Supplements)