Melatonin is widely treated as a harmless sleep aid, but several groups of people should avoid it or use it only under medical supervision. People taking blood pressure medications, blood thinners, or immunosuppressants face the most clearly documented risks. Pregnant and breastfeeding women, young children, and anyone with liver disease or an autoimmune condition also have good reasons to be cautious.
People on Blood Pressure Medications
One of the strongest warnings applies to people taking calcium channel blockers for high blood pressure. A controlled study of 47 patients taking nifedipine found that adding 5 mg of melatonin at bedtime raised systolic blood pressure by an average of 6.5 mmHg and diastolic blood pressure by 4.9 mmHg across the full 24-hour day. The blood pressure increase was most pronounced in the afternoon and early nighttime hours, with systolic readings climbing nearly 10 points during the first part of the night.
The likely explanation is that melatonin competes with the calcium channel blocker at a molecular level, weakening the drug’s ability to keep blood pressure down. For someone whose blood pressure is well controlled on medication, adding melatonin could quietly undo that control. If you take any blood pressure medication, this interaction is worth discussing with your prescriber before trying melatonin.
People Taking Blood Thinners
Melatonin can alter how your body processes warfarin (Coumadin), one of the most commonly prescribed blood thinners. Case reports and a published case series found that patients taking both melatonin and warfarin showed increases in INR and prothrombin time, two measures of how long it takes blood to clot. Higher values mean your blood is thinner than intended, which raises the risk of bruising or bleeding events. If you take warfarin or a similar anticoagulant, combining it with melatonin without close monitoring of your clotting levels is risky.
People With Autoimmune Conditions
Melatonin does more than regulate sleep. It actively stimulates parts of the immune system, boosting the production of certain white blood cells and increasing inflammatory signaling molecules. For most people this is harmless, but for anyone whose immune system is already overactive, it can cause real problems.
Case reports have documented flare-ups in patients with myasthenia gravis, autoimmune hepatitis, rheumatoid arthritis, and Crohn’s disease after starting melatonin. In the myasthenia gravis cases, melatonin appeared to rev up the exact immune pathways responsible for the disease while simultaneously making the patients’ immunosuppressant medications less effective. Melatonin triggered resistance to glucocorticoids in immune cells, meaning that even increasing the dose of steroids didn’t help. If you have any autoimmune condition, particularly one managed with immunosuppressants, melatonin could undermine your treatment.
People With Liver Disease
Your liver is responsible for breaking down melatonin. The hormone is processed through specific liver enzymes and then cleared through the kidneys. When the liver is damaged, particularly in cirrhosis, this breakdown slows significantly. That means a standard dose of melatonin can produce much higher and longer-lasting blood levels than it would in someone with normal liver function. This isn’t just a matter of feeling extra drowsy. Because melatonin affects blood pressure, immune function, and blood sugar, elevated levels over prolonged periods can amplify all of those effects in unpredictable ways.
People With Diabetes or Blood Sugar Concerns
The relationship between melatonin and blood sugar is complicated, and that’s precisely why people with type 2 diabetes should be cautious. A genetic variant in one of the melatonin receptors (MTNR1B) has been identified as a risk gene for type 2 diabetes, and research shows that melatonin can impair glucose tolerance when it coincides with food intake. Melatonin appears to suppress insulin secretion, which is normally fine during sleep when you’re not eating, but becomes a problem if you snack late at night or eat on an irregular schedule.
Shift workers, people who eat late in the evening, and melatonin supplement users are all susceptible to this mismatch. If you take melatonin and then eat, your body may handle that food less efficiently. For someone already struggling with blood sugar control, this effect could meaningfully worsen glucose management. The research in this area is genuinely conflicting, with some studies showing melatonin helps and others showing it hurts, which is itself a reason for caution rather than reassurance.
Pregnant and Breastfeeding Women
There is very limited human research on melatonin during pregnancy and lactation. Because melatonin crosses the placenta and passes into breast milk, its effects on a developing baby are largely unknown. No major medical organization has endorsed melatonin as safe during pregnancy. The lack of evidence isn’t reassurance. It means the risks simply haven’t been studied well enough to rule them out. Most clinicians recommend avoiding it during pregnancy and breastfeeding.
Children and Adolescents
Melatonin is increasingly given to children for sleep problems, but there are unresolved concerns about its effect on sexual development. In the body’s natural timeline, melatonin levels drop as puberty approaches, and this decline may play a role in triggering the hormonal cascade that starts puberty. Supplementing a child with melatonin could, in theory, interfere with that signal.
No clinical study has directly tested whether long-term melatonin use delays or disrupts puberty in children. However, animal studies show that melatonin affects kisspeptin, a hormone that acts as a master switch for the reproductive system. One study found that melatonin initially suppresses kisspeptin but then, over longer periods, increases it, potentially stimulating the reproductive axis in unpredictable ways. Until more is known, the concern is reasonable enough that long-term use in prepubertal children deserves careful consideration.
People Scheduled for Surgery
Melatonin reduces the amount of anesthesia needed during surgery, which can create dosing problems for your anesthesiologist if they don’t know you’re taking it. Stanford Medicine’s preoperative guidelines list melatonin among supplements that should be stopped seven days before a surgical procedure. If you have surgery coming up, stop taking melatonin at least a week beforehand and mention your use to your surgical team.
People at Risk of Falls
Melatonin causes drowsiness, and in older adults this effect can linger into the morning hours, especially when liver function is reduced and the supplement clears more slowly. The sedation and potential blood pressure changes create a combined risk for falls, which in older adults can lead to serious injuries. Older adults with balance issues, a history of falls, or blood pressure that fluctuates when standing should weigh this risk carefully before using melatonin as a nightly sleep aid.