Thoros-B is a biotoxin which was being developed by Dr. Zev Cohen on Noveria. He claims it was based on "an exotic lifeform discovered on the frontier". Binary Helix wanted something that could kill the creature, but there was no profit in a bioweapon that only affected one rare species, and Cohen was ordered to develop it to affect more species.

An accident during quarantine infected several members of his team with the toxin. Cohen may ask Commander Shepard to fetch a cure from their lab.