[caeril]

The primary concern is its (potential) effect on natural production. Most human hormone production is governed by the hypothalamus-pituitary-adrenal axis. Internal feedback loops suppress natural production for extended periods of time if exogenous hormones are introduced.

One of the most common examples of this is exogenous testosterone. If you take it long enough, the LH/FSH feedback hormones drop, the leydig cells in your testes atrophy and go dormant, and your natural production could be suppressed forever. Nearly all retired bodybuilders (and other athletes who use gear) are dependent on TRT for life, for this reason.

The open question is whether introducing exogenous melatonin to patients whose natural production is already very high (as it tends to be in children) could lead to lifelong dependency on it. That's the concern.