Biological Warfare (Apocalypse)
Biological Warfare can be considered the ultimate human-designed weapon to fight the aliens. This gives you the Toxigun, a small, potent weapon, and Toxin A, a weak toxin, but better than nothing. Once you get a recharging weapon, you will NEVER go back to clip-based weapons, though, so you pretty much won't use this unless desperate to get through a heavily shielded opponent quickly. Toxin C, however, packs a punch.
Official Entry (for Toxigun): "An X-COM weapon designed to shoot high powered projectiles containing anti-Alien toxic fluids. It is designed to cause minimal harm to other targets and is not very good at penetrating armor."
Official Entry (for Toxin A): "Ammunition for Toxigun. It contains a lethal fast acting poison designed to target Alien life forms. It is not so effective against the more advanced bipedal Alien types."