Food and Drug Interactions
Last Updated: April 2, 2013
Most of us are well aware that foods like artery-clogging French
fries and sugar-drenched sodas aren't good for our well-being-but
we may not think twice about healthy staples like leafy-green
vegetables, fruit juice, and herbal supplements. However, for
individuals (and particularly seniors) who are taking certain
prescription medications, it's important to know that some
surprising dietary elements can pose potential risks.
Approximately half of Americans aged 65 and older take five or
more medications, many of whom mix prescription drugs, over the
counter medications and herbal supplements, according to a report
from the University of Florida College of Nursing. Add to this
statistic the fact that adverse drug reactions occur two to three
times more frequently in elderly patients, and many doctors,
researchers, and pharmacists see cause for concern.
While anyone who takes both dietary supplements and medicines
runs the risk of experiencing an interaction, changes associated
with aging-combined with regular mixing of medications-make seniors
more susceptible, says clinical consultant pharmacist Lawrence
Lemchen, who specializes in geriatrics in Bellevue, Washington.
"I've filled prescriptions for 17 medications [for one elderly
person]," says Lemchen. "When you're combining that many drugs and
you add anything else to the mix, the likelihood increases that you
are to cause an interaction of some sort."
Many of the food and drug interactions that have been identified
in studies and medical literature involve herbal supplements like
ginkgo biloba and St. John's wort, but certain foods and vitamins
can also trigger interactions. Grapefruit juice, for example, can
affect the blood-level concentrations of numerous drugs, including
certain blood pressure-lowering drugs, cholesterol-reducing drugs,
and cyclosporine (for the prevention of organ transplant
rejection). Other types of non-herbal supplements, such as
melatonin and S-adenosylmethionine, can also interfere with
prescription medications, according to the American Council on
Science and Health.
"Basically, a person who has significant medical problems needs
to be very careful when they take any over-the-counter medication
or herbal supplement because it definitely has the potential to
interact with prescription medication," says Lemchen. "The
important thing is just to be cautious."
Why do interactions happen?
When a drug is taken orally, it usually travels from the stomach
to the liver. Here, specific enzymes then break down and metabolize
the drug (or, in other words, process the chemicals and remove them
from your body). Introducing new substances at the same
time-whether they are other drugs, over-the-counter medications,
herbal supplements or even foods-can potentially interfere with
these enzymes. The potential consequences depend on which
substances are being mixed, but boil down to two basic risks:
either your medication can be rendered ineffective or its effect
can be unintentionally increased.
"You can potentially saturate the enzyme system so there's not
enough enzyme left over to metabolize your drug. Then you're
raising your dose without anyone knowing it," explains outpatient
pharmacist Howard Crabtree at Harborview Medical Center in Seattle.
"Or there's the opposite scenario in which drugs, herbal products
and foods act as enzyme inducers. Then you get an abundance of
enzymes that chew up everything in their path, causing
sub-therapeutic doses of both agents."
Although the Federal Drug Administration (FDA) released a public
health advisory in 2000 about the risk of potential drug
interactions with St. John's wort, for the most part its studies
have focused almost exclusively on the effects of mixing different
prescription drugs-which means that food and drug interactions
caused by dietary and herbal supplements are not well understood.
And while the potential risks of interactions should certainly not
be dismissed, it's worth noting that there is some disagreement as
to the extent to which vitamins and herbal supplements are
One 2004 study of conducted by the University of Pittsburgh, for
example, found that 94% of food and drug interactions did not have
serious consequences-a conclusion that surprised many health care
professionals. A more recent study published in the March 2006
medical journal Geriatric Nursing, however, tracked 58 elderly
women, who reported taking at least one herbal product while using
over-the-counter or prescription medications, and discovered more
alarming results. The study detected 136 drug interactions among
the women-41% of which were deemed high risk and 58% moderate risk.
Despite their different findings, researchers in both cases came to
a similar conclusion: because limited information on food and drug
interactions exists, communication between caregivers and patients
Which substances can cause potential interactions?
The following are just a few of the foods, herbs, and vitamins
that have been known to cause interactions with prescription
A recent Institute of Medicine Report, sponsored by the FDA,
found that Americans spend close to $16 billion a year on dietary
supplements. And while the use of herbal products among Americans
65 and older has risen dramatically since the late '90s, a 2005
national study found that 49% of elderly patients taking herbal
remedies had not reported their use of supplements to their
"It can get very confusing, especially for seniors," says
Lemchen. "It's already hard to keep track of as many as nine or ten
medications, then you see ads for these herbal remedies that make
all sorts of promises."
Herbal supplements are the most worrisome potential interaction
agents, adds Crabtree, primarily because they are very loosely
regulated in the United States. "At least with grapefruit juice,
for example, you know what it is and you know what the
concentration is in a given product because it's regulated by FDA,"
he says. "Herbal supplements come in any number of dosage forms and
concentrations and can vary from batch to batch. The box right next
to another on a shelf could have a totally different
In order to minimize potential dosage variations, Lemchen and
Crabtree both stress the importance of buying supplements from
reputable brands. There are a variety of herbs and herbal
supplements that can harbor potential risks, but common
interaction-causing culprits include St. John's wort, ginkgo
biloba, goldenseal, kava, ephedra, and garlic. Individuals taking
high blood pressure medications, anticoagulants,
diabetes medications, heart medications, monoamine oxidase
inhibitors (MAOIs), and drugs that affect the liver should be
particularly cautious when taking these supplements.
The exact chemical or chemicals in grapefruits that cause
interactions are unknown, but the citrus fruit's pulp, peel, and
juice can interfere with your enzymes and essentially increase the
dosage levels of certain drugs. Avoid grapefruit juice if you are
taking certain anti-seizure medications, antidepressants,
benzodiazepines, calcium channel blockers, anti-arrhythmic drugs,
pain medications, and impotence drugs, among others.
Vitamin K and Vitamin E
Vitamin K (found in vegetables such as spinach, kale, broccoli,
and Brussels sprouts) and Vitamin E (also in leafy green
vegetables, as well as vegetable oils and nuts) produce
blood-clotting substances and can reduce the effectiveness of
anticoagulants such as warfarin (Coumadin®). Because
warfarin is highly sensitive to interactions and changes in diet,
adds Lemchen, it is particularly important for individuals taking
the medication to monitor their dietary intake.
Zinc and Echinacea
Vitamins containing zinc and herbal supplements containing
Echinacea act as stimulants for the immune system, which can
interfere with drugs such as corticosteroids or cyclosporine that
are meant to suppress the immune system. Echinacea should also be
avoided when taking medications that affect the liver, such as
Arava® or Nizoral®.
When your doctor or pharmacist asks about your medications,
remember to include over-the-counter drugs, herbal supplements, and
vitamins in your response. Your medical providers should warn you
about potential food and drug interactions, says Lemchen, but it's
also imperative to keep track of your medications (and their
potential side effects) yourself-especially if you have more than
Ultimately, Crabtree and Lemchen both stress the importance of
being open with your medical providers, whether it means mentioning
new side effects or sharing what you may suspect is too much
information. "Don't leave anything out, even if you think it's
irrelevant," says Crabtree. "Let [your doctor or pharmacist] decide
what's relevant and what isn't."
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