Q. I have four young children, and for the past year we have been plagued with pinworms. Every couple of months, the entire family takes a medication to kill pinworms, only to have them return again. I follow the laundering and cleaning procedures recommended on the drug package insert, but to no avail.

Please explain the life cycle of the pinworm. For example, how long do the eggs survive outside the body, how long does the medication take to kill the worms, and how long is the medication effective? Also, are there any side effects of using this medicine -- Vermox -- so frequently? Do you have any suggestions on how to resolve our problem?

A. Pinworms can be a distressing nuisance and sometimes difficult to get rid of. Fortunately, this condition hardly ever leads to any serious illness.

Pinworm infection is very common, occurring in children much more often than in adults. As many as one in five children will be bothered by this parasite. Between 30 and 40 million people in the United States get this infection, many without even knowing it.

Pinworms are tiny. The female is about 10 millimeters long -- about three-eighths of an inch. The male is even smaller -- only 3 millimeters. They gain access to the body through the mouth, usually on fingers contaminated by pinworm eggs. The eggs are most often picked up by touching contaminated skin around the anal area.

Sometimes, the eggs are transferred from contaminated clothing, sheets or close contact with others in group settings such as day care or school. Outside the body, the eggs can remain alive and able to cause infection for two to three weeks.

Once inside the body, the eggs soon hatch. The newborn worms then travel through the intestines where they take up residence. Within a month, female worms are mature. At night, they exit the body through the anus, lay thousands of microscopic-size eggs around the anal area, and die.

The major symptom of pinworms, anal itching, is caused by the exiting female worms. The itching can make children irritable and cause them to lose sleep. Girls can sometimes be bothered by vaginal itching or discharge as well.

Once you suspect that your child has pinworms, your doctor should be contacted to arrange for testing. Once the condition is confirmed, treatment is usually straightforward.

Two good remedies for pinworms are Antiminth and Vermox. Each is taken in a single dose. All members of a household should take one of these medicines at the same time, to avoid reinfecting someone after they've been treated. The medicine works the first day you take it but doesn't stay around long enough to prevent new worms from reinfecting your children if they're exposed to them.

Although it's often recommended that you wash clothing and bedsheets to remove the eggs, this is probably not as crucial as treating all close contacts and preventing reinfection from contact with infected people outside the home.

Pinworm medications are generally safe, even with repeated use, but they should usually not be taken during pregnancy, especially during early pregnancy. Most people don't have any side effects, although some may get an upset stomach. In repeated cases of infection, it's wise to repeat treatment about two weeks after the first dose.

Because you've had a problem with recurrent infections, I'd recommend that you look into the possibility that your children are bringing the infection home with them. It may take treating all their contacts in day care or school to put an end to the problem.

Consultation is a health education column and is not a substitute for medical advice from your physician. Send questions to Consultation, Health Section, The Washington Post, 1150 15th St. NW, Washington, D.C. 20071. Questions cannot be answered individually.