Q. I've been feeling tired, and my girlfriend said I should do a colon cleanse to detoxify my body. There are lots of great customer testimonials, and the ads say it would eliminate toxins and buildups that clog the large intestine (colon), strengthen the immune system and improve absorption of vital nutrients. The Web sites say an unhealthy digestive system impacted with mucus, toxins and waste can cause fatigue, bad body odor, obesity, backaches, bloating, arthritis and mood swings. There are lots of gross-looking photos of the stuff that comes out after you start taking the capsules. Shall I try it?

A. First see your doctor to see if anything else is causing your fatigue. Colonic cleanser capsules usually contain herbs and botanicals such as flax seed, fennel seed, licorice root and aloe vera. Cleansers also often include laxatives such as senna and cascara, magnesium compounds, or fiber like psyllium (which is what Metamucil is). Some use oxygen releasers.

However, there's no scientific studies showing that colonic cleansers improve health, reduce fatigue or help you lose weight.

Testimonials are considered the weakest level of scientific proof, even with hundreds of such postings. Just do a Google search on "evidence-based medicine" to find out why.

To see whether colonic cleansers work, you need randomized studies that compare two groups: a control group that takes fake capsules, and a second group that takes the real McCoy, and this has never been done.

In fact, tinkering with your normal gut metabolism may even worsen things. Losses of potassium, sodium and other trace minerals because of colon cleansers may cause fatigue or heart rhythm problems. And in the March 2003 issue of the Proceedings of the National Academy of Sciences, Dr. Giovanni Pitari and others from Thomas Jefferson University and the Mayo Clinic found that a toxin from some strains of E. coli, a bacteria that normally lives in the gut, also stifles the growth of cancerous intestinal cells. This may be why colon cancer strikes people less often in developing countries such as India, where toxin-producing strains are more common. Vitamin K is another product produced by bacteria in the intestines.

Perhaps nature intended there to be a natural mix of bacterial byproducts in the gut. So I wouldn't recommend short-circuiting the natural wisdom of your body by the use of colon cleanser capsules.

Dr. Landis Lum is a family-practice physician for Kaiser Permanente and an associate clinical professor at the University of Hawai'i's John A. Burns School of Medicine. Send your questions to: Prescriptions, Island Life, The Advertiser, P.O. Box 3110, Honolulu, HI 96802; islandlife@honoluluadvertiser.com; or fax 535-8170. This column is not intended to provide medical advice.