Q: I'm 50 years old and have been constipated most of my life. For the past few months I seem to have diarrhea for a day or two followed by constipation for a week or so. What could cause this?

A: A change in bowel habits is a warning sign of colon cancer, so I would recommend getting checked.

Some other things that can cause diarrhea alternating with constipation are:

*Irritable bowel syndrome, also known as irritable colon or "nervous stomach."

*Using laxatives inappropriately.

*Antacids, some of which may cause constipation, others diarrhea.

*Severe constipation that causes a partial blockage in the colon around which loose stool occasionally passes.

*Inflammation of the colon, which occurs, for example, in ulcerative colitis, Crohn's disease and diverticulitis.

For a complete check, your doctor inspects your lower intestines with a tube inserted in your rectum (sigmoidoscopy), followed by a barium enema, an X-ray examination of the entire colon. Your doctor may also test your stool for trace amounts of blood, a sign of polyps, cancer or inflammation in the intestines.

Q: I'm a 30-year-old woman and just had several chest X-rays and a mammogram. Does breast tissue permanently absorb radiation? I would like to have another baby and breast-feed. Is it possible that the breast or breast milk would still be a source of radiation to the baby?

A: Breast tissue does absorb radiation during the instant it's exposed to a chest X-ray or a mammogram (breast X-ray), but there is no lasting radiation in the breast or breast milk that could harm your baby.

The same would be true even if you were currently breast-feeding -- it would still be safe to breast-feed after either of these tests. Breast tissue, and other areas of the body for that matter, don't become radioactive after routine X-rays.

This also applies to CAT scans, the newer MRI (magnetic resonance imaging) scans and ultrasound (also known as sonograms).

The situation is different for scans known as radionuclide scans. To do these types of scans, a technician injects a small dose of a radioactive solution into a vein, then takes pictures with a scanning device. The radioactivity is very short-lasting and, depending on the substance used, usually fades within one to three days. In some cases, it may last as long as one to two weeks. During this time, breast-feeding should be stopped.

If you need to have one of these scans done while you are breast-feeding, you should pump your breasts beforehand and store up enough milk to last the time breast-feeding is not permitted. In most cases, however, there are alternative imaging techniques that can be substituted for radionuclide scans, so you shouldn't have to face this problem.

Q: What's the difference between acute and chronic prostatitis, both in terms of symptoms and treatment?

A: Actually there are three major kinds of prostatitis, inflammation of the prostate gland in men: acute, due to a bacterial infection, and two kinds of chronic prostatitis -- one with a bacterial infection and one without. The chronic, nonbacterial infection may be the most common.

Acute bacterial prostatitis is a sudden, severe infection of the prostate gland causing fever, chills, pain in the lower back and crotch, burning and difficulty with urination, and strong, frequent urges to urinate. This infection usually responds to antibiotics, which you may need to take for up to a month.

With chronic bacterial prostatitis, you generally have less severe problems with pain and urination, and no fever or chills. In some men, the only sign may be a persistent urine infection detectable by culture but causing few symptoms. With this kind of prostatitis, you need to take antibiotics longer, for up to three months, to make certain you clear the infection.

Chronic prostatitis without any infection is the most difficult kind to correctly diagnose and successfully treat. Its cause is unknown. Symptoms are similar to those of chronic bacterial prostatitis, but no infection can be found. Although no therapy is consistently effective, different treatments include hot baths, regular sexual activity and prostate massage done by your doctor.