Overdoses of methadone -- used as a replacement drug for opiate addicts but also an increasingly popular painkiller -- can lead to heart and breathing problems and death, the Food and Drug Administration (FDA) says in a new warning.

The Associated Press reported Nov. 27 that the warning notes that methadone can cause slow or shallow breathing and potentially deadly changes in users' heart rate. Dangerous side effects and deaths have been reported among people who use the drug as a painkiller.

Part of the problem, the FDA said, is that while methadone is effective as a painkiller for 4-8 hours, the drug can stay in the body for up to 59 hours. Patients who take subsequent doses to relieve pain can unwittingly build up toxic levels of the drug in their bodies.

The FDA urged doctors to be cautious about prescribing the drug and monitor patients closely, giving strong warnings to users against taking more of the drug than prescribed.

Doctors wrote more than 2 million prescriptions for methadone as a painkiller in 2003, and use of the drug is still rising. An estimated 2,452 overdose deaths were attributed to methadone in 2003, up from 623 in 1999.