There is insufficient evidence to show that new opioid antagonists are effective in treating constipation due to strong analgesics. Opioids (morphine-like drugs) are substances used to treat severe pain. They cause many side effects, and frequently affect the stomach and bowels. Opioid-induced bowel dysfunction (OBD) is a recently coined term used to describe constipation, incomplete evacuation of the bowels, bloating, and increased reflux of stomach contents. OBD occurs both with short and long term use of opioids, in patients with many types of diseases, and causes increased disease and reduced quality of life. Traditional opioid antagonists (drugs that block the receptors which opioids bind to) can be used to reverse many of the side effects of opioids, including constipation, but because they block opioid receptors in the brain, they may also reverse reduction of pain. The search for opioid antagonists that act only in the gut ("peripherally active" antagonists), therefore not reversing reduction of pain, has produced two new drugs, methylnaltrexone and alvimopan, which are at an advanced phase of development. There is not enough information to make firm conclusions about the safety or effectiveness of traditional opioid antagonists in the treatment of OBD. Alvimopan and methylnaltrexone both show promise in treating OBD, but further data will be required to fully assess their effectiveness.