Q. I've had a problem with my leg for the past year or two. Every day I have to walk four blocks to and from work. After I've walked about half this distance, my leg feels very tight and hard. On some days it's my whole leg and on some just the calf. It hurts to the point that I have to stop for a while. I've been told that this could be due to a need for potassium. I tried taking potassium, but after a month of using it there was no improvement. What could be causing this problem?

A. You may have something doctors call intermittent claudication, from the Latin word for limping.

Claudication is an attack of leg pain and momentary difficulty in walking. It is caused by hardening of the arteries in the legs (atherosclerosis), most often causing problems in the calf muscles.

Narrowed arteries lead to decreased blood flow. During exercise, the oxygen needs of your leg muscles cannot be met by the impaired blood supply, and this produces a charley horse-like cramping, aching or tightness in the affected leg. A clue to this diagnosis is that your symptoms repeatedly occur after a set amount of exercise.

To check for this problem, your doctor may feel for pulses in your legs and feet, and measure the blood pressure in your legs. If your doctor suspects a problem with your circulation, he or she may order an arteriogram, an X-ray of the blood vessels in your legs.

If you have a blockage, there are several options. Bypass surgery is the main treatment. In this procedure, a surgeon replaces blocked blood vessels with artificial grafts or transplanted veins. If you have a large but localized blockage, it may be possible to open it up by inflating a balloon catheter that's been inserted through a needle in the skin. If surgery isn't possible, you may benefit from Trental, a new medicine used to help blood flow through narrowed blood vessels in the legs.

Other things you can do to help this problem are avoid smoking, maintain normal weight, lower your blood cholesterol if it's high, and get regular exercise as prescribed by your doctor.

Q. My doctor informs me I have gallstones and recommends surgery. Recently, I read there is a new medicine to dissolve gallstones. Why should I have surgery?

A. Chenix -- the only gallstone dissolving medicine now available -- is not for most people. It does work in some cases, but there are enough problems with using it that most people are better off with surgery.

Chenix is really chenodiol, a substance normally found in the bile juice inside the gallbladder. In certain people with certain types of gallstones, taking Chenix for long periods of time slowly dissolves the stones. This drug works best in people who: *have a single gallstone; *have gallstones made of cholesterol. Chenix doesn't dissolve calcified stones or stones made of bile pigments. *Have gallstones that float. This can be determined only with an X-ray known as an oral cholecystogram. *Have small stones, about a half inch or less. *Are not healthy enough to undergo surgery. *Are not pregnant.

These restrictions severely limit the number of people likely to benefit from treatment. For example, fewer than one in five people with gallstones have stones that float.

Treatment is long, up to two years of taking pills twice a day. Chenix causes liver damage severe enough to force stopping the drug in about 3 percent of people taking it. Diarrhea, a common side effect, is mild in 30 to 40 percent of people and severe enough to prompt lowering the dose in 10 to 15 percent or stopping the drug altogether in 3 percent. Even in the right types of patients, whose stones dissolve completely, gallstones return half the time within five years after finishing treatment.

Higher doses of Chenix seem to work better, but cause more side effects. Lower doses actually may lead to more attacks of gallbladder pain and higher rates of surgery than no treatment at all.

Treating people with Chenix is expensive, time-consuming and far from straightforward. What we really need is a better medicine for gallstones.

A drug similar to Chenix, ursodeoxycholic acid, seems to work better with fewer side effects. It is currently undergoing testing. Two other drugs, monooctanoin and methyl tertiary butyl ether, are also being tested, but these must be given through a tube inserted into the gallbladder while you are in the hospital.

Follow-Up

Q. In your answer about the artificial sweetener aspartame, you didn't mention whether it's okay to drink products made with NutraSweet while you're pregnant. I'm pregnant now, and I'm concerned about the risk to my unborn baby.

A. There's no evidence that aspartame has caused damage to unborn babies, but the maximum recommended amount of aspartame that you can safely consume is lower if you're pregnant.

Normally, the maximum safe blood level of phenylalanine, one of the two amino acids that make up aspartame, is 100 micromoles per deciliter. But phenylalanine becomes concentrated in the fetus, whose blood level is about double that of the mother. So the maximum safe level in pregnant women is 50 micromoles per deciliter. To reach that level, you'd have to drink between 68 and 85 12-ounce cans of aspartame-sweetened soda at one time.

Taken in moderation, aspartame doesn't raise your body's level of phenylalanine above normal, and doesn't appear to cause serious problems to mothers or their unborn babies.