In House M.D., Gregory House is addicted to Vicodin.
Vicodin is a prescription painkiller combining hydrocodone and paracetamol, also called acetaminophen. House uses it to manage chronic pain caused by an infarction in the quadriceps muscle of his right leg. His limp and cane are visible consequences of that injury.
The addiction is one of the show's major continuing storylines. House hides or minimizes his dependence, while colleagues including James Wilson and Lisa Cuddy repeatedly confront its effects on his judgment, relationships, and employment. His attempts to stop using the drug become especially important in later stories.
House's pain condition and his addiction are related but not identical. The infarction explains why he experiences chronic leg pain; Vicodin is the medication he misuses. Other pain medicines appear in the series, but Vicodin is the signature drug associated with House.