Mehmet Oz, M.D., is host of “The Dr. Oz Show,” and Mike Roizen, M.D., is Chief Wellness Officer and Chair of Wellness Institute at Cleveland Clinic. Email questions to youdocsdaily@sharecare.com.

QUESTION: Does grapefruit juice affect levothyroxine effectiveness?

ANSWER: There are about 100 drugs that can have dangerous and even life-threatening effects if you eat grapefruit or drink grapefruit juice while taking them, but levothyroxine, a thyroid hormone drug used to treat an underactive thyroid, is not one of them.

Grapefruit juice may, however, slightly reduce the body’s ability to absorb the drug, so if you often drink grapefruit juice, levothyroxine levels may be lower than normal, impairing treatment.

Dozens of other medications — including commonly used drugs such as certain statins, some new non-warfarin oral blood thinners, opioids such as oxycodone, oral fentanyl, methadone and some cancer drugs — can be extremely dangerous if you drink grapefruit juice, said David Bailey, professor emeritus of clinical pharmacology at the University of Western Ontario who first described the interaction and periodically updates a list of affected drugs maintained by the Canadian Pharmacists Association.

“You don’t have to drink liters and liters of the stuff to have an effect,” Bailey said. For example, he said, “if you take simvastatin and drink a single glass of grapefruit juice, it’s like taking three times the dose,” though the impact can be much more or much less, since individual susceptibilities vary widely.

Other citrus fruits like Seville oranges, limes and pomelos can also produce a similar effect.

Additional drugs affected by grapefruit are some AIDS medications, certain birth control pills and estrogen treatments, antihistamines, Viagra, some heart drugs like quinidine, certain antipsychotics and gastrointestinal agents, some immunosuppressant drugs and some blood pressure drugs.

Under normal circumstances, these drugs are metabolized in the gastrointestinal tract, where an enzyme called CYP3A4 deactivates them, so relatively little of the active ingredient is absorbed.

But grapefruit contains natural chemicals called furanocoumarins that inhibit the enzyme, and without it the gut absorbs much more of the drug, causing blood levels to spike.

The result can be kidney failure, fatal respiratory depression, gastrointestinal bleeding or a life-threatening heart arrhythmia, depending on the person and the drug.

The effect of grapefruit on levothyroxine is the opposite: Drinking grapefruit juice — or orange juice — can inhibit uptake transporters and reduce the drug’s absorption. The effect does not last as long but can lead to under-treatment of the thyroid condition.