Avoiding Drug Interactions

How to safely take prescriptions and reduce interactions with food, alcohol and OTC drugs.

Many of us are getting better at asking questions about how the drugs we are prescribed work or what their side-effects might be. And yet we often remain uninformed, or at least unsure, of how the drugs we take interact with the food we eat, over-the-counter drugs we also take, or the alcohol we (may) drink.

Nearly half of US residents took at least one prescription drug within a 30-day period in 2012, according to the CDC. Americans also rely heavily on over-the-counter products, as the average household will spend $340 this year on these often cheaper, non-prescription remedies, says the Consumer Healthcare Products Association (CHPA). And yet, despite being a highly medicated society, the overwhelming majority of Americans do not seek advice about how to take their medications properly — frequently resulting in undesirable consequences like severe side effects, dangerous drug interaction, inability to manage an illness, or even, occasionally, death. Drug interactions are often responsible for some these unwanted reactions, and that risk obviously rises the more drugs a person takes.

Here is a guide designed to clear up some commonly held misconceptions about drug interactions involving food, OTC drugs and alcohol.

Food Plus Prescription Drugs

What you eat and drink affects how the drugs you take work in your body, in a couple of ways. First, certain foods may interfere with a particular drug’s active ingredients, rendering it less effective. Second, foods can make a drug’s side effects more pronounced. Two common food-drug interactions are here:

Blood Thinners and Green Vegetables

greensVsBloodThinnerMany people believe that they should avoid all green vegetables if they are taking blood thinners. This is simply not true. Green vegetables don’t interact with all blood thinners, but they just happen to interact with the most commonly prescribed blood thinner in history, warfarin (Coumadin or Jantoven).

“It’s a myth that warfarin patients should stay away from these foods,” says Devon Trumbower, PharmD, BCPS, an internal medicine clinical pharmacist with the Christiana Care Health System in Wilmington and Newark, DE. “Green veggies like spinach, kale, and broccoli are high in vitamin K, which can antagonize warfarin’s ability to anti-coagulate or ‘thin’ the blood,” she notes. A more accurate recommendation for green vegetables and blood thinners is to be consistent with how much high-vitamin-K-containing foods you eat while on warfarin. That means, for example, about a serving or so a day, rather than going on a kick of an all-veggie diet one week, and then eating no green veggies the following week.

So while the amount of vitamin K in your food may interact with your warfarin, it doesn’t mean you have to avoid green vegetables altogether. But, what it does mean is that you might:

  • Want to discuss adding leafy greens to your diet with your doctor
  • Be referred or ask your doctor for a referral to an anti-coagulation clinic
  • Need a dose change if you’ve altered your diet, which means you may have to get your blood checked a few times until your doctor pinpoints your new dose

Maybe (with your doctor’s okay) you really can have your greens and eat them too!

Grapefruit and Many Drugs

grapefruitVsDrugsAnother controversial food is the otherwise very nutritious citrus fruit, grapefruit. Many people believe that they should avoid grapefruit altogether — especially when taking certain drugs like statins or calcium channel blockers. However, the effect grapefruit has on medication depends on many factors — including the amount of grapefruit or grapefruit juice you drink, how often you drink it, and how much time elapses between consuming grapefruit and taking your medication.

“Grapefruit and grapefruit juice contain chemicals that can inhibit the enzymes that break down certain types of medications in the intestines,” explains Trumbower. “When a medication does not get broken down properly in the intestines, you can have too much medicine in your blood, which in turn, increases your chances of having side effects. People can be affected differently depending on the amount of enzymes their bodies make.”

Genetics also plays a role. Some people break down a drug at what’s considered a “normal” rate, while others have genes that cause them to process drugs faster. If you’re a faster metabolizer, you may need a higher dose of a drug to get its therapeutic benefit, because the drug doesn’t hang around in your system long enough to do its job. But if you are a slower metabolizer, you might have more of a drug in your system for a longer period of time. It’s those slower metabolizers who are most at risk for problems linked to grapefruit.

Also, if a drug is metabolized by the liver via the same enzyme as grapefruit, grapefruit consumption can further slow the elimination of the drug, which can cause the drug to accumulate in concentrations that exceed the therapeutic dose and cause severe side effects. Since these effects vary from person to person, it’s probably a good idea to ask your doctor first, who may recommend a different drug or different dosage.

Over-the-Counter (OTC) Meds Plus Prescriptions

OTCVsPhrmaAside from convenience, OTC medications offer many benefits — the most notable of these being cost savings. Not only are OTC medications lighter on your wallet, they can also save tax dollars and lower the cost of healthcare. In fact, the U.S. health care system can save as much as an estimated $7 (again according to stats from the CHPA) for every single dollar spent shopping the OTC aisle.

Even though OTC products account for less than 10% of sales in the pharmaceutical industry, they still represent an $80 billion a year industry. Not only that, but nearly 80% of Americans rely on over-the-counter products to help manage aches, pains, fever and other complaints.

Because the overwhelming majority of Americans use OTC medications, they can be a cause for concern. Why? The CDC estimates that there are more than 300,000 OTC products on the market in the United States, and many of them contain either similar or identical ingredients. Using two or more OTC products that contain either the same or related ingredients might result in an accidental overdose. The risk doesn’t stop there: The active ingredients found in some OTC drugs are also found in some prescription drugs. And sometimes, the prescription doses are higher, which could potentially increase both the risk and the severity of drug interactions.

A good rule of thumb for choosing over-the-counter medications — especially those with multiple active ingredients — is to check with your doctor or pharmacist prior to purchasing the product, she says. Acetaminophen (Tylenol) is the first example to come to mind, and it bears repeating, because so many of us rely on OTC acetaminophen for headaches and other pain, and then think nothing of, for example, taking a cough or cold med without realizing that these drugs also often contain the drug. A good rule of thumb to avoid taking too much, says Trumbower: check labels and be sure you never take more than 3000 mg of acetaminophen a day unless under the direction of their doctor.

“People need to understand that just because something is over-the-counter doesn’t mean that you don’t need to take precautions, or that there won’t be adverse effects,” says Frank Cantrell, PharmD, professor of clinical pharmacy at the University of California San Francisco College of Pharmacy, and director of the San Diego Division of the California Poison Control System. “People assume that if they’re taking an OTC drug, these drugs are safe and have no adverse effects, and that they can take the drug however they want,” he says.

Over-the-Counter (OTC) Plus More OTC

OTCvsManyOTCDrugsAnother common problem with OTCs, says Cantrell, is that people often assume that the benefit of taking these drugs increases by taking more. “Just because a little bit works, taking a lot doesn’t mean that it will work better,” he maintains. “As a rule of thumb, don’t take more than the recommended amount.”

Laxatives are a classic example. Pharmacists have observed that patients seem more likely to overuse OTC laxatives than prescription ones. Many people, especially those 65 and older or who take opioid-containing medications for pain suffer from chronic constipation. Other constipating drugs include the heart drug verapamil (Calan, Verelan), and depression medications like amitriptyline (Elavil), and clonidine (Catapres). Dehydration and low fiber intake can also contribute to chronic constipation.

The problem occurs when constipation sufferers overuse senna-containing stimulant laxatives (such as Sennakot and Sennakot-S) and bisacodyl (such as Dulcolax). What you might not know is that senna and bisacodyl generally are not safe for long-term use and should be reserved for acute situations of severe constipation and occasional use, as frequent use may actually cause electrolyte disturbances.

Some laxatives, like the fiber-based Benefiber and Metamucil, are safe for long-term use. People who are constipated for five days or more with no major improvement should see their doctors right away.

Alcohol Plus Prescription Drugs

wineVsPillTo drink or not to drink: That is the question. Most medical professionals warn their patients against drinking while taking pretty much any drug — prescription or OTC. But how realistic is that for most of us? It’s hard to pass up that glass of wine that pairs well with a favorite meal. But will having one little drink hurt, and if so, how much? Well, that depends on many factors, like a person’s age, gender, liver condition, whether the person ate, the type of food consumed and, of course, the drug

Many drugs, says Trumbower, have some of the same effects as alcohol. “Many prescription medications, including those for depression, anxiety, pain, muscle spasms, high blood pressure, seizures, and allergies, can cause some degree of sleepiness or dizziness. Even a small amount of alcohol can amplify these effects, which can make driving unsafe and increase the risk of falling or having an accident.”

Cantrell points out that labels and other literature provided when purchasing drugs over-the-counter or via prescription offer helpful hints. “In general, look at the warnings to find out whether you should abstain or avoid alcohol. Any combination of drugs that causes sedation has a multiplying effect, meaning that instead of 1+1=2, 1+1=3,” he cautions.

“Drinking alcohol while taking high doses of acetaminophen puts patients at high risk of liver damage, that’s a completely different issue than taking alcohol with sedating medication,” explains Cantrell.

The easiest way to avoid drug interactions, he says, is to get to know your pharmacist. “People sometimes forget that we are specialists in all medications — including over-the-counter (OTC) drugs. Bottom line: If you have concerns or considerations, you should check with your pharmacist first.”

So, don’t be afraid to read labels on your OTC drugs or review the patient handouts that come with your prescriptions. And finally, get to know your pharmacist. They are happy to chat — and help.

Further Reading


Frieda Wiley

Frieda Wiley

Frieda Wiley is a medical writer and consultant pharmacist who calls the serene Piney Woods of East Texas home. Her work has appeared in Arthritis Today, Diabetes Self-Management, Everyday Health, and Next Avenue. She also develops content for medical students and allied healthcare professionals.


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