Dear Dr. Roach: My family doctor said to stay away from multivitamins because they contain metal contaminants. Is this true, and is it good advice? — J.K.

Answer: Your doctor is correct that many multivitamins have been shown to be contaminated with heavy metals. For example, in a recent article on prenatal vitamin and folic acid supplements, 80% contained quantifiable lead, while 15% had a greater amount than 0.5 mcg per serving (the limit from California’s Proposition 65).

Unfortunately they did not specify which brands are the ones without detectable heavy metals, but the article noted that supplements also containing calcium or iron were more likely to be contaminated. The article noted that products using an independent lab (such as the United States Pharmacopeia or the National Sanitation Foundation) to test purity were the safest.

Even though lead can be detected at very low levels, the magnitude of the risk at these small levels is thought to be small. Still, it is worthwhile considering what the benefits of multivitamins are against the potential risk.

Since many large, well-done studies have been published that do not show any consistent benefit to healthy adults taking multivitamin supplements, even a small amount of potential harm may make taking them not worth it.

However, for prenatal vitamins with folic acid, the benefit in preventing neural tube defects is absolutely worth it, so I would look for third-party-certified products when they really are needed, such as before and during pregnancy.

Dear Dr. Roach: Is the Naltrexone shot (for alcohol abstinence) more effective when given on one side of the body compared to the other? A person I know who receives the shots feels that it’s only effective when they receive it on their right buttock. Is this possible, or could they be noticing a placebo effect? — C.M.

Answer: Naltrexone is an effective treatment for people with alcohol dependence. The exact effectiveness depends on the setting, but people who use naltrexone are less likely to relapse on their drinking.

Of course the drug will be absorbed just as well from the left side as the right. However, our expectations can have a huge influence on the effectiveness of a treatment. When there is an expectation of benefit, there is a placebo effect. (When there is an expectation of harm, it’s called a nocebo effect.) The placebo effect isn’t as critical in a situation like antibiotics for pneumonia but can be of immense importance when the desired effect is a decrease in alcohol cravings.

If I were this person’s doctor, I would keep giving the injection on their right side.