DEAR DR. ROACH: My boyfriend, who is in his 60s, got the recommended shingles shot a couple of years ago. Three weeks ago, he got a very bad case of shingles. It went from the front of his stomach, along the left side, to the middle of his spine. He has huge blisters, a red rash, pain and no appetite, and he constantly sleeps. The doctor put him on a regimen of valacyclovir. He was told to keep the area moist to avoid cracking and pulling of the blisters and rash. I thought that if you got the shingles shot, you wouldn’t get the shingles, or at least you’d get only a mild case. His has not been a mild case. The doctor told him that the shot just gives a false sense of security. If so, why get the shot? — D.H.

ANSWER: I disagree with his doctor; it isn’t a false sense of security so much as it’s an incomplete sense of security. No medical treatment is 100 percent effective, and that includes vaccines. Even the best vaccines don’t work on some people, or can’t be used, which is why having a large proportion of the population vaccinated is so critical for a really infectious disease, like measles.

In the case of shingles, the vaccine prevents about 50 percent of shingles episodes from ever occurring, and it might keep others from being worse. (As an aside, it’s possible that your boyfriend’s case would have been even worse without the vaccine, but there’s no way of ever knowing for sure.)

What might be the most important reason of all to get the vaccine is to prevent the dreaded complication of shingles — post-herpetic neuralgia. The rate of post-herpetic neuralgia in vaccinated people who still get shingles is 67 percent lower than in nonvaccinated people.

It’s not a perfect vaccine, but it’s a lot better than no vaccine. Its downside is seldom more than a redness at the injection site or a sore arm, and it has never been shown to transmit the virus. Post-herpetic neuralgia, on the other hand, can be exquisitely, disablingly painful and can last for months.

DEAR DR. ROACH: I was told that due to the way our food is processed and grown, most people lack magnesium and should take a magnesium supplement. Should I? I read that one symptom of low magnesium can be constipation or irregularity. — C.B.

ANSWER: The prevalence of low magnesium depends on the population. About 2 percent of the general population has low magnesium levels, but among people with diabetes, the rate is about 25 percent. In people who abuse alcohol, the prevalence may be as high as 30 to 80 percent. Some diuretics used for high blood pressure predispose a person to magnesium deficiency. High magnesium foods include leafy green vegetables, nuts, cereals and avocados.

The most common symptoms of severe low magnesium are loss of appetite and muscle tremors and weakness. Magnesium is necessary for potassium and calcium balance, so these can be deficient in cases of low magnesium. Since only a small amount of body magnesium is in the blood, magnesium deficiency should be considered in people with unexplained low calcium and potassium.

Low magnesium is rare in nondiabetic people who eat a good diet. I don’t recommend magnesium supplementation for low-risk people with no symptoms. Symptoms of low magnesium should be evaluated by a physician and not self-treated. Only those with diagnosed low magnesium should take a magnesium supplement.

Dr. Roach regrets that he is unable to answer individual letters, but will incorporate them in the column whenever possible. Readers may email questions to ToYourGoodHealth@med.cornell.edu or request an order form of available health newsletters at 628 Virginia Dr., Orlando, FL 32803. Health newsletters may be ordered from www.rbmamall.com.