Constipation is very common. Like headaches and colds, almost everyone will experience a bout at some point. For most, these episodes are brief and usually don't require medical treatment. But doctors start to get concerned if the frequency of your bowel movements drops below three per week for at least two weeks. At any one time, an estimated 35 to 45 million people in the U.S. are in that predicament, or have been diagnosed with chronic constipation or a condition called Irritable Bowel Syndrome (IBS).

If you are constipated, the first thing to do is eat more fiber-rich foods such as vegetables, fruits, whole gains, legumes and beans. If that doesn't work, try a fiber supplement. Our analysis found that those containing psyllium are best. If your constipation persists for two to three weeks, you may need a drug. A variety are available, both nonprescription and prescription. These include stool softeners and different types of laxatives. The vast majority of people will get the help they need from inexpensive nonprescription drugs.

Taking cost and the evidence for effectiveness and safety into account, we chose the nonprescription drug polyethylene glycol (MiraLax) as our Best Buy. This drug improves constipation symptoms and is just as effective as a prescription drug called lactulose. In addition, it has a track record of being safe and well tolerated by most people. We caution against the long-term use (beyond a few days) of the laxatives senna (Senokot, Ex-Lax) and bisacodyl (Correctol, Doxidan, Dulcolax). Studies indicate these drugs are less effective against chronic constipation.

If you're unable to tolerate MiraLax or it doesn't improve your symptoms, you should talk to your doctor about other options, which may include generic lactulose or lubiprostone (Amitiza). Warning: Amitiza is a new and expensive prescription drug which may be no more effective than MiraLax or lactulose for most people and its safety profile is not fully established.

Amitiza is also approved for women who have IBS with constipation as the main symptom (and many doctors will prescribe it for men too). But we advise people with this condition to talk to their doctors about trying other medicines first, in addition to lifestyle changes, and only consider Amitiza if these strategies fail to provide relief.

This report was released and last updated in December 2008.

*Selected doses are listed. Also, not all brands or branded generics (of which there are many) are included.