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Q: How often do I need full-mouth X-rays from my dentist? I want to minimize my radiation exposure.

A: : Every five years or so, but it depends on your oral health and history, your age, and whether you have any symptoms or signs of dental problems. You will also need less extensive "bitewing" X-rays every 12 to 24 months. If your oral health is good, bitewings every 24 months or so will be enough. But if you have many fillings, crowns, and bridges and don’t take good care of your teeth, you’ll need more frequent bitewings.

There are three types of dental X-rays. Bitewings are used primarily for the molars; they show the exposed teeth and only part of the roots. Periapical X-rays show the whole tooth, from crown to root tip, and some of the bone surrounding the roots. "Full-mouth" X-rays include these two kinds. In addition, panoramic X-rays, which provide a wide view of the upper and lower jaw, are used if you have jaw problems, cysts, or impacted teeth, and if you’re going to have a tooth extraction.

The radiation exposure from today’s dental X-rays is very small, but there’s no point in getting X-rays you don’t need. In addition, you should wear a lead apron and collar during the procedure. Pregnant women should postpone X-rays, if possible.

Q: Does drinking lots of lemonade prevent kidney stones?

A: Yes, in some people this may reduce the risk of a recurrence. In general, increasing your fluid intake helps.

There are several different kinds of kidney stones, and a low level of citrate in the urine increases the risk for two common types: calcium-oxalate and uric-acid stones. If you’ve had one of these kinds of stones, your doctor may prescribe potassium citrate to raise your citrate level. In addition, or instead, you may be advised to eat lemons, which are rich in citrate, or drink lemonade. Citrate binds to calcium in the urine, preventing the formation of crystals that can develop into kidney stones. It also affects the acid-base balance of the urine, which plays a role in stone formation.

Lemons contain almost five times more citrate than oranges. Half a cup of pure lemon juice provides the amount of citrate your doctor is likely to prescribe. In one study, most people with stones who drank eight glasses of lemonade a day (containing 4 ounces of reconstituted lemon juice, in total) ended up with normal levels of citrate in their urine after six days. This should help prevent stone recurrence.

If you’re worried about the calories, use a sugar substitute to sweeten your lemonade, or buy artificially sweetened lemonade. Just make sure it contains real lemon juice (or citric acid).

But not grapefruit juice: Some studies have found that grapefruit juice actually seems to increase the risk of kidney stones. Thus, if you have a history of stones, your doctor may suggest that you avoid, or at least limit, this juice.

Q: What causes bloodshot eyes? Should I worry about them?

A: When eyes are "bloodshot," the small vessels on the surface of the eyeballs are dilated and visible. Many people notice that their eyes are bloodshot first thing on awakening. Lack of sleep, alcohol consumption the night before, overuse of contact lenses, or an allergy are among the possible causes. Bloodshot eyes during the day can be caused by some irritant, eyestrain, excessive rubbing, or anything that tends to dry your eyes (high heat, low humidity, or insufficient tear production).

Most commonly, if the cause is external and temporary, bloodshot eyes improve by themselves. Washing your face and eyelids with cold water can help, and so can cold compresses. Over-the-counter eye drops (which usually contain a decongestant to constrict blood vessels) and eyewashes may help, too. But follow instructions carefully with any eye product. Overuse of some eyedrops can actually increase redness. And eyecups can be a source of infection, so use only the disposable kind.

Colds, flu, and hay fever can produce bloodshot eyes as well, but occasionally the condition can be a sign of more serious disease or injury. Conjunctivitis, or "pinkeye," which usually looks more alarming than bloodshot eyes, is an infection that produces very red, irritated eyes, along with itching and a discharge. If you think you have an eye infection, if your eyes don’t clear up, or if you have eye pain or changes in vision, you should seek medical advice.

UC Berkeley Wellness Letter, November 2004