I was on antibiotics for a bladder infection during my pregnancy and then given clindamycin during delivery because I tested positive for strep. After delivery, I developed colitis, which was said to be antibiotic-related. My doctor put me on a lactose-free diet instead of antibiotics because I am nursing. The diet seems to have helped, but I am wondering how long it will be before this condition clears up. Will I need to watch out for lactose in my diet indefinitely? What is the connection between lactose and the colitis? Are there any long-term effects?

T.R.

Most cases of diarrhea related to antibiotics are not due to infection. The diarrhea is simply a side effect of the antibiotic itself. However, an infection called antibiotic-associated enterocolitis is caused by the bacterium Clostridium difficile. The way C. difficile causes disease is quite interesting. When someone takes certain antibiotics, the normal populations of microorganisms in the bowels are altered. This gives C. difficile, which is not killed by those antibiotics, an opportunity to multiply exponentially. Sometimes, C. difficile produces a toxin that damages the lining of the small and large intestines, causing abdominal pain and diarrhea. Usually, this is a mild infection that resolves once the antibiotics are discontinued. Occasionally, the infection is quite severe, resulting in high fevers, bleeding, other infections and rarely death. In the more serious cases, physicians give oral antibiotics to kill C. difficile. This typically results in total resolution of the illness.

I suspect that this infection was the cause of your colitis. The antibiotic clindamycin (trade name Cleocin) is a common culprit in C. difficile-associated disease.

Lactose is a sugar found in dairy products. We digest lactose with the help of an enzyme called lactase, which is found in the lining of our small intestines. If there is a deficiency of lactase, lactose travels through the small intestine undigested, and bacteria in the colon break it down. The byproducts of the bacterial digestion result in gas, bloating and diarrhea. People who have a lactase deficiency (called "lactose intolerance") can avoid these problems by consuming special lactose-free dairy products or taking pills containing the lactase enzyme.

Many people with lactose intolerance inherit the condition; it typically becomes apparent later in life. You were told to avoid lactose because any process that damages the lining of the small intestine can also result in lactose intolerance. Various infections, including C. difficile enterocolitis, commonly cause this problem. Giardiasis, a form of infectious diarrhea caused by a parasite that contaminates water, is the classic disease that results in lactose intolerance. In some cases of giardiasis, the lactase deficiency is permanent, though a few months of deficiency is more the rule.

When I see a patient who has unexplained diarrhea and bloating that started with typical C. difficile colitis or another infection, I instruct the patient to avoid all lactose-containing products for one week. If all the symptoms resolve, I have the patient slowly add dairy products to the diet one at a time. Usually by two months' time, the diet is back to normal. If you are not having fevers or abdominal pain, then the infection has likely resolved. On a lactose-free diet, you should be better in less than a week. If your diarrhea persists, or if you do have fever or abdominal pain, you should be tested for C. difficile. If the test is positive, then treatment with oral antibiotics may be justified. You may need to stop nursing for a short period to avoid passing the antibiotics to the baby through breastmilk.