Can melatonin affect testicle size?

Yes, melatonin can affect testicle size. It has been noted in experiments with Siberian hamsters that increasing the darkness, thereby increasing melatonin due to reduced light exposure, can result in significant shrinkage of their testicles to the size of a grain of rice 1. This effect has been linked to melatonin's influence on the reproductive axis, where in females of the same species, melatonin can cause them to leave estrus.

Furthermore, a noted concern highlighted by another expert, Matthew Walker, is testicular atrophy or shrinkage in juvenile male rats when high levels of melatonin are given. This phenomenon poses concerns, particularly when considering the dosing of melatonin in pediatric populations, which has been seeing an increase 2.

Dr. Andrew Huberman discusses these effects in the context of experiments with animals and expresses reservations about using melatonin supplements, especially due to the often superphysiological levels contained in these supplements compared to the body's natural production 1.