The romulus potion was invented and patented by Peter Romulus, and it allows a werewolf to withstand the effects of the full moon. The components to create the potion are incredibly difficult and expensive to acquire, and the potion does not have a long shelf-life. This oftentimes makes it unobtainable for werewolves who are already trying to keep their affliction secret.
Oftentimes when acquired, it is delivered in two parts. The potion and a small vial of activator so that the potion is not yet finished (and not yet beginning to expire) until right before use.