Propoxyphene (Darvon) is an analgesic (pain killer) that has been used in the medical field for many years. It was first introduced as an alternative that was stronger than over-the-counter drugs but not as potent as substances such as morphine. Propoxyphene works by inhibiting neurotransmitters connected to OP3 receptors, thereby dulling physical pain. After being used as a prescription drug for a number of years, it was revealed that propoxyphene caused heart failure to its users. In November of 2010, the drug was recalled across the U.S. and banned from the public market.