A patent medicine was a medicinal compound that was advertised and sold as a cure for an ailment; however, the medicine never worked as advertised.
After Captain James T. Kirk returned from Rigel XII, in 2266, following the successful acquisition of much-needed lithium from the miners, Doctor Leonard McCoy jokingly asked him if he had considered working in the patent medicine business, as he must have given "quite a talk down there". Kirk declined, asking why he ought to "work your side of the street". (i.e. medicine) (TOS: "Mudd's Women")