Propoxyphene with acetaminophen, or Darvocet N 100, is a combination drug used for mild to moderate pain. It is commonly abbreviated as Propo-N/Apap on prescription labels. It contains 100 mg of propoxyphene napsylate and 650 mg of acetaminophen. Acetaminophen is more widely known by the brand name Tylenol.
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In 2004, propoxyphene was the 12th highest-selling drug in the United States. Propoxyphene was banned for use in the UK in 2005, and it is rarely used in Canada. Propoxyphene has about half the pain-relieving power of a similar dose of codeine. It is a Schedule IV controlled substance in the United States. The Public Citizen consumer group has been working to have propoxyphene removed from the United States market.
Propoxyphene does not cause the stomach upset that codeine does, and it can be given to people who are allergic to codeine and morphine. Acetaminophen relieves pain and reduces fever. Unlike other pain relievers, acetaminophen does not cause upset stomach.
For pain control, propoxyphene with acetaminophen is less effective that 400 mg of ibuprofen (Motrin). Because it is a mild pain killer, many people assume that it is benign. The toxic dose, however, is not much higher than the dose needed for pain relief. As few as 6 tablets can be fatal if taken at once. According to the FDA, propoxyphene has been a factor in 10,000 deaths in the United States since the 1950s. Of the deaths associated with propoxyphene between 1981 and 1999, 38.6 percent of them were accidental. There is no good antidote for propoxyphene overdose. Acetaminophen is safe in recommended dosages, but high doses harm the liver.
Propoxyphene relieves pain by two mechanisms. It is an opioid-receptor agonist, which means that it stimulates the opioid receptors. This lessens the body's ability to feel pain. Propoxyphene also blocks the N-methyl-D-aspartate (NMDA) receptors. The NMDA receptors transmit pain signals in the body.
Acetaminophen blocks the synthesis of prostaglandins in the central nervous system. Prostaglandins send pain signals and cause fever.
High doses of propoxyphene can cause seizures and coma. It can also slow breathing to a dangerous extent. These effects are increased when propoxyphene is taken with alcohol. During propoxyphene overdose, the drug is metabolized, or broken down, to a substance that is harmful to the heart.
People taking propoxyphene should not take more than one tablet every four to six hours. Because the toxic dose is only slightly higher than the dose needed for pain control, you should not take more than directed.