Is taking melatonin once per week ok

The short version: taking melatonin once a week is almost certainly fine for most people, but the evidence for any benefit is weak and the long-term effects of regular use are not well studied.

The evidence you got is a mixed bag. One small trial in children with eczema found that 3 mg of melatonin for 4 weeks improved sleep onset by about 21 minutes and modestly reduced eczema severity [4]. A meta-analysis in older adults having surgery found that perioperative melatonin reduced the odds of delirium by about 37% [5]. But these are narrow, short-term uses. For a healthy adult taking it once a week for general sleep, the data is almost entirely missing.

The pharmacokinetics are well understood. Melatonin is cleared from the body quickly, with a half-life of about 30 to 60 minutes [1]. A single weekly dose will not accumulate. The body's own production is about 30 micrograms per day [2], while typical supplements are 0.3 to 5 mg, which is 10 to 150 times the natural amount. A 0.3 mg dose produces blood levels around 220 pg/mL, which is within the normal nighttime range [6]. A 3 mg dose pushes levels to over 1,300 pg/mL, far above what the body ever produces naturally [6]. That high dose also lowered core body temperature in one study [6], which is a physiological effect, not necessarily a harmful one, but it shows the body is not indifferent to it.

The real question is not acute toxicity. It is what happens when you regularly override the body's own circadian signaling. The pineal gland produces melatonin in response to darkness. Taking it exogenously tells the brain that the signal has already arrived. Over time, this could blunt the body's own production, though the evidence for this is thin and mostly from daily use, not weekly. The 2014 systematic review on older adults recommended the lowest effective dose to mimic physiological rhythms [3], which is a cautious position.

The safety data is short-term. The eczema trial reported no adverse events in 4 weeks [4]. The delirium meta-analysis covered only the perioperative period [5]. No study in this retrieval followed people on weekly melatonin for months or years. That does not mean it is dangerous. It means nobody has done the work.

My call: for a healthy adult, taking melatonin once a week at a low dose (0.3 to 1 mg) is unlikely to cause harm, but the benefit is uncertain and the long-term effects of regular use, even weekly, have not been studied. Confidence: moderate for short-term safety, low for long-term safety.