Interaction studies have only been performed in adults.
Pharmacokinetic interactions
• Melatonin's metabolism is mainly mediated by CYP1A enzymes. Therefore, interactions between melatonin and other active substances as a consequence of their effect on CYP 1A enzymes are possible.
• Caution should be exercised in patients on fluvoxamine, which increases melatonin levels (by 17-fold higher AUC and 12-fold higher serum Cmax) by inhibiting its metabolism by hepatic cytochrome P450 (CYP) isozymes CYP1A2 and CYP2C19. The combination should be avoided.
• Caution should be exercised in patients on 5- or 8-methoxypsoralen (5 and 8-MOP), which increases melatonin levels by inhibiting its metabolism.
• Cigarette smoking may decrease melatonin levels due to induction of CYP 1A2.
• Caution is advised in patients taking cimetidine, since this agent increases plasma melatonin levels by inhibiting its metabolism by CYP1A2.
• Caffeine increases the concentrations of both endogenous and orally administered melatonin by inhibiting CYP1A2 catalysed melatonin metabolism.
• Caution should be exercised in patients on oestrogens (e.g. contraceptive or hormone replacement therapy), which increase melatonin levels by inhibiting its metabolism by CYP1A1 and CYP1A2.
• CYP1A2 inhibitors such as quinolones may give rise to increased melatonin exposure.
• CYP1A2 inducers such as carbamazepine and rifampicin may reduce plasma concentrations of melatonin.
• There is a large amount of data in the literature regarding the effect of adrenergic agonists/antagonists, opiate agonists/antagonists, antidepressant medical products, prostaglandin inhibitors, benzodiazepines, tryptophan and alcohol, on endogenous melatonin secretion. Whether or not these active substances interfere with dynamic or kinetic effects of melatonin or vice versa has not been studied.
Pharmacodynamic interactions
• Alcohol should not be taken with melatonin, because it reduces the effectiveness of melatonin on sleep. Alcohol can impair sleep and potentially worsen certain symptoms of jet-lag (e.g. headache, morning fatigue, impaired concentration).
• Melatonin may enhance the sedative properties of benzodiazepines and non-benzodiazepine hypnotics, such as zaleplon, zolpidem and zopiclone. In a clinical trial, there has been clear evidence for a transitory pharmacodynamic interaction between melatonin and zolpidem one hour following co-dosing. Concomitant administration resulted in increased impairment of attention, memory and co-ordination compared to zolpidem alone.
• Melatonin has been co-administered in studies with thioridazine and imipramine, active substances with affect the central nervous system. No clinical significant pharmacokinetic interactions were found in each case. However, melatonin co-administration resulted in increased feelings of tranquillity and difficulty in performing tasks compared to imipramine alone, and increased feelings of “muzzy-headedness” compared to thioridazine alone.
• Caution is advised in patients taking nifedipine, since concurrent use of melatonin and nifedipine may increase blood pressure. Concomitant use of melatonin and warfarin may lead to enhanced anticoagulation – INR should be checked when used together. Melatonin may also enhance the effect of direct-acting anticoagulants (e.g. dabigatran, rivaroxaban, apixaban, edoxaban).