Things that ordinarily ought to be good for us can turn dangerous in a hurry when we’re taking strong medicines.
Fruit juices, particularly grapefruit, but also orange and apple juice, can not only dramatically increase absorption of some drugs, creating the potential for overdosing, but can also block the uptake of other prescriptions, potentially wiping out their benefits.
Drugs known to be over-absorbed with grapefruit include many cholesterol-lowering statin drugs, antianxiety drugs and over-the counter cough medicines. Many drug labels carry warnings about drinking grapefruit juice while taking them.
One of the more extreme examples of apparent grapefruit interaction was reported earlier this month in the British medical journal Lancet.
A woman in the Olympia, Wash., area was hospitalized last November with a severe blood clot in her left leg that doctors believe formed because she had days before started a crash diet that included eating about half a pound of grapefruit each morning.
The woman had rarely eaten the fruit before, was taking an oral contraceptive, had an inherited disorder that put her at higher risk for clots and had just gone on a long car trip. Her doctors think the tipping point was that the large amount of grapefruit she was eating blocked an enzyme that normally breaks down estrogen.
The drug-absorption blocking effect of the juices is a more recent discovery, made by researchers at the University of Western Ontario, the same group that first noted increased absorption nearly 20 years ago.
The culprit seems to be a component of the juice called naringin, which blocks a key drug uptake transporter that helps move drugs from the small intestine into the bloodstream.
“We’re concerned about the loss of benefit of medications essential for treatment of serious medical conditions,” said David Bailey, a professor of clinical pharmacology who led the study, first reported last summer during an American Chemical Society meeting.
The test involved healthy volunteers taking the antihistamine fexofenadine with either water, water mixed with naringin, or grapefruit juice. They found that those who took the drug with grapefruit juice absorbed half as much as those who took their dose with a glass of water.
So far, grapefruit, orange and apple juice (which have substances similar to naringin) have been shown to lower the absorption of etoposide, a cancer-fighting drug, a group of beta blockers used to treat high blood pressure and heart disease, the immune-suppressing transplant drug cyclosporine and some antibiotics, like ciprofloxacin and levofloxacin.
“This is just the tip of the iceberg,” Bailey said. “I’m sure we’ll find more drugs that are affected this way.” Bailey said he feels that until all the testing is done, it’s probably a good idea to take most medications only with water and avoid washing meds down with fruit juices.
Other research on vitamin C and cancer drugs, done in labs on cancer cells and in mice, shows that high levels of the vitamin actually make chemotherapy 30 per cent to 70 per cent less effective, depending on the drug tested.
Scientists at Memorial Sloan-Kettering Cancer Center in New York reported the findings last October in the journal Cancer Research. They then implanted cancer cells, both pretreated with vitamin C and not, into mice. All were treated with anticancer drugs. In animals with cancer cells pretreated with vitamin C, tumors still grew rapidly, while those not pretreated seemed to remain stable.
So, while other studies have demonstrated that antioxidants like vitamin C are effective at preventing genetic damage to cells that can lead to cancer, the same protection works against drugs that are trying to destroy cancerous cells.
“Vitamin C appears to protect the mitochondria from extensive damage, thus saving the cell,” said Dr. Mark Heaney, an associate attending physician who led the research. “And whether directly or not, all anticancer drugs work to disrupt the mitochondria to push cell death.”
Heaney said the findings don’t mean cancer patients should avoid vitamin C, particularly in their regular diet, which should include foods rich in the vitamin. But he cautioned that taking large supplemental doses of the vitamin combined with chemo is probably counterproductive.
Likewise, researchers from several institutions reported last fall that women taking aromatese inhibitors, a class of drug that blocks estrogen production, to treat breast cancer or prevent recurrence, might undermine the drugs’ effectiveness by taking a soy-based dietary supplement, genistein.
The inhibitor drugs work by blocking estrogen production, which the body still makes from other hormones even after menopause. Since genistein contains estrogen from plants, the effect of the anticancer drug is reduced and tumours can begin growing again.
Lee Bowman writes for the Scripps Howard News Service.