Can Grapefruit Juice Affect My Thyroid Medicine?

Q. Does grapefruit juice affect levothyroxine effectiveness?

A. 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 like certain statins, some new non-warfarin oral blood thinners, opioids like oxycodone, oral fentanyl, methadone and some cancer drugs — can be extremely dangerous if you drink grapefruit juice, said Dr. David Bailey, a professor emeritus of clinical pharmacology 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,” Dr. 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.

“Despite the concern for toxicity, which can be very serious in some cases, just exposure to grapefruit juice will not likely produce an adverse clinical reaction in most patients,” Dr. Bailey said. “The greatest hazards may come from consuming grapefruit repeatedly during the day, which can cause a further increase in blood levels of affected drugs.” Other citrus fruits like Seville oranges, limes and pomelos can also produce a similar effect,

“Significant adverse events generally were documented for patients doing well on a stable drug regimen who included grapefruit at a normal or large amount for a few days to a week,” Dr. Bailey said. “Other than that, it was not possible to predict ahead of time which patient would get into difficulty. However, the problem can be totally preventable by avoiding grapefruit consumption in every case or prescribing an equivalent drug that is not affected by grapefruit.”

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.

Levothyroxine tablets should be taken only with water and on an empty stomach first thing in the morning, and you should wait a full hour before eating or drinking anything else, said Dr. James Hennessey, director of clinical endocrinology at Beth Israel Deaconess Medical Center in Boston. “Anything but water can potentially lead to malabsorption of levothyroxine,” he said.

Supplements and other drugs can also interfere with absorption. You should wait at least four hours after taking levothyroxine before taking antacids or vitamins or supplements containing iron or calcium. Regular consumption of foods that contain soy, walnuts or dietary fiber can also reduce the absorption of levothyroxine.

If you’re concerned about whether you are achieving the right blood levels of a medication, check with your doctor to see if your dose needs adjustment.

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