Both the alien and adapted biogel have a 50% chance to cause one of ten random effects that lasts 30 seconds. Using multiple biogels simultaneously can cause more than one effect, but overlapping effects will not stack (you can suffer from -1 PE, +2 AG, -1 IN all at the same time, but -1 IN and -1 IN does not become -2 IN).
The ten "Biochemical Anomaly" effects are as follows:
This makes the adapted biogel by far the best healing item in the game. It surpasses stimpaks in healing, as well as a 25% chance for a temporary positive side effect (50% chance for a side effect, and a 50% chance that effect is positive). Even if a negative side effect is acquired, the radiation and deductions are barely noticeable. Lowered Intelligence does not reduce skill points when you level up, nor does reduced Endurance lower your HP (it just reduces resistances).
Adapted biogel can only be obtained from Elliot Tercorien, by asking him to convert any alien biogel you have. This option is available from your first encounter with him onwards, including after completing This Galaxy Ain't Big Enough.... Because of this, and because one of the random items Sally can give the player (when asked if she's found anything on the ship) is alien biogel, the potential supply of adapted biogel is effectively unlimited.