Gill Man DNA is a powerful mutagen that is incorrectly believed by its creator to be a steroid.
Ingested in the form of steam, Gill Man DNA is derived from mutated fish DNA, itself engineered from several different fish species and then further warped by exposure to the magical energies of the Sunnydale Hellmouth.
Although it does increase the swimming abilities of those who take it, the manner in which this is achieved is by slowly mutating them into amphibious fishlike humanoids (it is not clear whether they can breathe underwater as well as in the open air).
Related Drugs: Black Frost, Calendula, Calynthia Powder and Orpheus.