I’m taking a supplement that contains grapefruit-seed extract, which purportedly fights infections. Is there any good evidence to support that claim? —J.L., Ottawa, Ontario, Canada

No. A few studies have found that disinfectants made with grapefruit extract can kill certain bacteria, yeasts, and molds. But that effect probably comes from preservatives in the sprays rather than the extract, since comparable preservative-free products don’t seem to kill the bugs. Moreover, like the fruit itself, grapefruit-seed extract blocks a key drug-metabolizing enzyme, which may lead to increases in the blood levels, risks, and side effects of numerous drugs. If you take medications, talk to your doctor before you start routinely eating grapefruit or taking a supplement that contains it—or before stopping if you already do, which could cause a sudden drop in the levels of medication in your blood.