Melatonin generates singlet oxygen on laser irradiation but acts as a quencher when irradiated by lamp photolysis.

Melatonin, a naturally occurring chemical mediator, although assigned a diverse range of functions, has attracted interest in recent years because of its ability to function as a free radical scavenger. Because of the implications of singlet oxygen in neurotoxicity, the objective of the study was to investigate the ability of melatonin to quench singlet oxygen generated using laser irradiation or lamp photolysis. The results show that melatonin produces radicals upon laser irradiation while the lamp photolysis studies show that melatonin is able to scavenge singlet oxygen produced by naphthalene. While melatonin is a free radical scavenger under biological conditions, it acts as a generator of singlet oxygen and or radicals (as PhiDelta is 1.41) when irradiated with laser light, implying that it has the potential to be used in photodynamic therapy in the destruction of tumors.