Milk and other calcium products, for example, can block the absorption of certain antibiotics. And eating large quantities of chocolate while taking some antidepressants can cause a sharp rise in blood pressure. Even some licorice can reduce the effects of certain blood pressure drugs and diuretics.
A recently published review in the Canadian Medical Association Journal that found an increasing number of prescription drugs could have potentially dangerous interactions with grapefruit and grapefruit juice highlights the importance of consumers being aware of possible food and drug interactions.
It's been known for some time that grapefruit juice can "both increase or decrease the absorption of a small number of drugs," says Hartmut Derendorf, chairman of pharmaceutics at the University of Florida College of Pharmacy. Derendorf was not involved in the Canadian review.
"If the drug is metabolized in the gut wall to a large extent and this metabolism is blocked, then the concentrations in the blood will go up. An example is the lipid-lowering drug simvastatin (Zocor)," says Derendorf. "For other drugs such as the antihistamine fexofenadine (Allegra), grapefruit juice blocks the uptake into the bloodstream and the concentrations in blood will go down."
In all such cases there are alternative medications available that will not interact with grapefruit and the other citrus fruits that contain furanocoumarins, the culprit behind the "grapefruit juice effect," Derendorf says. These include Seville oranges (often used in marmalade), limes and tangelos, a cross between tangerines and grapefruit. Sweet oranges, such as naval and Valencia, do not contain furanocoumarins.
There's more attention than ever before being paid to reducing interactions between food and drugs, Derendorf says, and researchers are focused on finding safe alternatives, removing the chemical compounds that cause interactions, and in some cases, changing the genetic makeup of foods so that they do not interfere with medications.
In December, a team at the University of Florida's Citrus Research and Education Center published a paper about ongoing efforts to create a grapefruit hybrid with significantly reduced interaction risk.
Concerns about the interactions between foods and drugs are not limited to prescription medications. Dietary supplements (which also include vitamins, minerals and herbs) can likewise interfere with how some medications work.
"Often, people think that herbal supplements are benign and don't have interactions, and that's absolutely false," says Christine Gerbstadt, a registered dietitian and spokesperson for the Academy of Nutrition and Dietetics. "Most supplements have the exact same chemicals and ingredients as pharmaceutical drugs but simply in a naturally occurring form or lower dose," she says.
Drugs, of course, can have negative interactions with other drugs, and the same can be true of supplements. Calcium supplements, for example, may decrease the absorption of dietary iron, which is why people at high risk for iron deficiency are encouraged to take calcium supplements at bedtime, instead of with meals.
When being prescribed any medication it's important to inform your physician of all medications you're currently taking - prescription, over-the-counter and dietary supplements - to read the patient information material that comes with the medication and to ask about potential interactions, Gerbstadt says.
"Your pharmacist and registered dietitian can also provide a wealth of information," she says. "No one person can know it all, but huge databases are available to help professionals get access to this information."
A sampling of the potential interactions between foods and medications, including some supplements, according to the Food and Drug Administration:
-- Grapefruit juice: Some statin drugs to lower cholesterol, such as simvastatin (Zocor), atorvastatin (Lipitor), and pravastatin (Pravachol); some blood pressure-lowering drugs, such as Nifediac and Afeditab ; some organ transplant rejection drugs, such as Sandimmune and Neoral (both cyclosporine); some anti-anxiety drugs, such as BuSpar (buspirone); some anti-arrhythmia drugs, such as Cordarone and Nexterone (both amiodarone); some antihistamines, such as Allegra (fexofenadine); the anti-malaria drugs Quinerva or Quinite (quinine); and Halcion (triazolam), a medication used to treat insomnia.
-- Licorice: The sweetening compound glycyrrhizin in licorice may reduce the effects of some blood pressure drugs or urine-producing drugs including Hydrodiuril (hydrochlorothiazide) and Aldactone (spironolactone). It may increase the toxicity risks from Lanoxin (digoxin), used to treat congestive heart failure and abnormal heart rhythms.
-- Chocolate: Antidepressant Monoamine Oxidase (MAO) inhibitors (such as phenelzine (Nardil, Nardelzine) and tranylcypromine (Parnate) are just one category of drugs that shouldn't be consumed with excessive amounts of chocolate and other caffeinated foods. Caffeine can also interact with stimulant drugs such as Ritalin (methylphenidate), increasing their effect, or by decreasing the effect of sedative-hypnotics such as Ambien (zolpidem). Using bronchodilators with caffeinated foods and drinks can increase the chance of side effects, such as excitability, nervousness, and rapid heart beat.
-- Potassium-rich foods (such as bananas, oranges, and green leafy vegetables): Can add to high potassium levels in the body caused by ACE (Angiotensin Converting Enzyme) inhibitors including captopril (Capoten) and enalapril (Vasotec) prescribed to lower blood pressure or treat heart failure. Too much potassium can cause an irregular heartbeat and heart palpitations.
-- St. John's Wort (Hypericum perforatum): Can reduce concentrations of medications in the blood, including digoxin (Lanoxin), used to treat congestive heart failure and abnormal heart rhythms; the cholesterol-lowering drug lovastatin (Mevacor and Altocor), and the erectile dysfunction drug sildenafil (Viagra).
-- Vitamin E: Taken with a blood-thinning medication such as warfarin (Coumadin) can increase anti-clotting activity and may cause an increased risk of bleeding.
-- Ginseng: May increase the risk of bleeding when taken with anticoagulants (blood thinners such as warfarin and heparin). Can also increase the bleeding effects of aspirin and non-steroidal anti-inflammatory drugs such as ibuprofen, naproxen, and ketoprofen. Combined with MAO inhibitors such as Nardil or Parnate may cause headache, trouble sleeping, nervousness and hyperactivity.
-- Ginkgo biloba: High doses can decrease the effectiveness of anticonvulsant therapy in patients taking seizure-control medicines Tegretol, Equetro or Carbatrol (carbamazepine), and Depakote (valproic acid).
