In fact, in 1972, after clinical studies qued propoxyphene's effectiveness as a pain reliever, Lilly was required to release a statement backing off some of its earlier claims on behalf of the drug. Some authorities, including the American Medical Association's Drug Evaluations, went a step further, arguing that a dose of propoxyphene is actually less effective than a normal dose of aspirin. When people run into problems with propoxyphene, it's often due in large part to the drug's ineffectiveness. Since it's most often prescribed for relief of pain, propoxyphene can easily be misused-particularly when relief does not appear to be fast or forthcoming. But pain sufferers who self-prescribe a double dose of Darvon when a first dose fails may be writing themselves a prescription for trouble.
Reasons revolve around unusual properties of the drug itself.
For one thing, the gap between a therapeutic dose and an overdose is small. This margin of safety is so slight, in fact, that as little as four times the standard dose can trigger a dangerous slowing of breathing and heart rate. Six times a therapeutic dose can cause seizures and symptoms of toxic psychosis. Darvon's small safety margin shrinks further if the drug is taken with alcohol or other depressants. Given propoxyphene's similarities to methadone, it's not surprising that the drug is as addictive as it is.