Metaphasic radiation is a form of energy which, under certain conditions, reverses the aging process of most humanoid species, thereby prolonging the life of those who are exposed to it. Metaphasic Radiation also has remarkable regenerative properties, as in the case of Geordi La Forge's eyes, and also has rejouvernating properties, increasing the metabolism and energy levels of the crew of the USS Enterprise (NCC-1701-E) temporarily.
The only danger associated with Metaphasic Radiation is that if an organism is exposed to Metaphasic Radiation for extended periods of time, (Presumably ten Earth years)and then the organism is removed from that enviornment, the organism's body starts breaking down at the cellular level, and rapid aging is the result, as in the condition of the Son'a.
High concentrations of metaphasic radiation were discovered to be contained within the rings of the Ba'ku planet in the Briar Patch in 2375. ("Star Trek: Insurrection")