Surprising Facts of Commonly Prescribed Drug Interactions: What You Need to Know.

Are you taking more than one medication? Or are you taking prescribed medication(s) with over the counter medications, nutraceuticals , or supplements?  If so, you may be at increased risk for drug interactions, which occur when a drug, a supplement, or even a food product affects the way your body processes a medication. Is this probably why you are not seeing any beneficial effect of one or more of your current prescribed medications? Or is it drug interactions that sometimes cause us to react unfavorably to certain medications?

 What is Drug Interaction? The term "drug interactions" is somewhat a misnomer. The reason being that drugs don't combine in the body to produce chemical reactions. Instead, a drug, supplement, or food may affect how long a medication stays in the body, often by stimulating or inhibiting the production of specific enzymes in the liver or intestine. These enzymes are part of the cytochrome p450 system, which plays an important role in metabolizing (or handling) many drugs.  Such interactions can make a drug more powerful-so that a standard dose becomes an overdose - or can render it less potent or altogether ineffective.  Now drugs interactions usually occur in one of the following ways:

Interactions with other drugs: Interactions between two drugs occur when one drug affects the cytochrome enzyme that process the other. For example, the antibiotic metronidazole (Flagyl) inhibits the enzyme CYP2C (part of the Cytochrome P450 system) , which breaks down warfarin (Coumadin). If the two medications are taken together, the blood thinner (coumadin) can stay longer in the body, thereby increasing the risk of severe bleeding. Another example is the anti-seizure drug phenytoin (Dilantin) which stimulates excess production of CYP3A4, the enzyme that metabolizes estradiol, a component of low-dose contraceptives. In this case, the contraceptives are cleared more rapidly in women taking phenytoin, reducing their effectiveness and increasing the chance of an unplanned pregnancy.

There are other instances, whereby  two drugs taken for different purposes may have the same effect, producing something like an overdose. For example,  when over-the-counter pain relievers- aspirin, ibuprofen (Motrin), naproxen (Aleve), and to a lesser extent, acetaminophen (Tylenol)-are taken with blood thinners like warfarin and clopidogrel (Plavix). Since the pain relievers also have anti-clotting effects, the combination can result in severe bleeding as well.

Interactions with nutrients: Surprisingly, several foods can also block or stimulate the enzymes that break down drugs. People who take atorvastatin (Lipitor) or simvastatin (Zocor), with large amounts of grapefruit juice may experience muscle pain and other side effects from “overdose,” because the juice inhibits the enzyme that clears the statins (anti-cholesterol medications). Fish oil supplements can have a similar effect when taken with warfarin, increasing the risk of severe bleeding. Iron supplements can diminish the effects of levothyroxine (Synthroid), the medication used to treat an underactive thyroid. The same reason why your physician instructs you to avoid grape juice, and some vegetable when you are on warfarin.

Interactions with your body: Your body itself may react to drugs in unexpected ways. There is more than one version of each of the 50 or so CYP450 enzymes, and there is no easy or reliable way to identify which ones you have inherited. You may have reactions to certain drugs because you break them down faster or slower than most people do. Also, increasing age, liver or kidney disease tends to metabolize drugs more slowly, so that a dose lower than the one usually recommended may be enough.

How do we then minimize the risk of drug interactions? Being proactive in your own health, checking for drug interactions, and discussing concerns with your healthcare provider can be a life-saving task.


Bottle of prescription drugs


Knowing why you are taking each medication is key: Drug names are often hard to pronounce, difficult to remember, and easy to mix up. An error when you list your drugs could mean a potential interaction will go unnoticed. Consider labeling each pill bottle or package with the reason you’re taking the drug-for example, “blood pressure.”

Educate yourself on how to take the medication: It's important to learn whether to take your medication with food or on an empty stomach. And some drugs are not to be taken with specific foods. For example, the antibiotic tetracycline shouldn't be taken with dairy products because calcium interferes with the drug's absorption. Following the instructions on how to take your medication as indicated on the prescription bottle becomes very essential.

Go easy on grapefruit juice: While it's true that grapefruit juice affects the metabolism of several drugs, it usually takes about a quart of the juice to make a difference. Also, the use of grape juice may also have untoward effects on medication like Viagra used for erectile dysfunction.


Glass of grapefruit juice


Use the same pharmacy to fill your medications: The health care system is still fragmented. Your primary care team is likely to have a record of the prescriptions you've gotten from that office, as are the specialists you have seen. However, each isn't likely to know what the others have prescribed. Although pharmacies store records of all prescriptions they fill, one pharmacy may not have access to the records of another and so may not have a complete record of your medications. Keeping an updated list of your medications would be very helpful, especially in emergency situations.

Beware of the use of supplements: Some of the most serious drug interactions involve prescription medications and supplements. Not only are supplements less likely than FDA-approved medications to be listed in the databases of drug interactions, but health care providers also may not know what supplements people are taking. Since there isn't much evidence that supplements have health benefits, it's best to avoid them unless your doctor prescribes them.

Limiting your alcohol intake: It isn't a good idea for women to have more than a drink a day in general, and it can be even worse to drink while you're taking drugs. Combination of alcohol with certain drugs will cause severe  drowsiness-an intended effect of sleeping pills, and a side effect of many antihistamines, antidepressants, and anti-anxiety medications. Also, combination of alcohol with medications used for Opioid use disorder such as suboxone, methadone, sublocade is extremely dangerous. One should also exercise extreme caution when combining alcoholic drinks with insulin or oral antidiabetic agents (pills) due to potential  very low blood sugar (hypoglycemia) it might cause.

Talk to your healthcare provider: Communication with your healthcare provider is key in helping to prevent drug interactions. Keep an up-to-date list of your medications, over-the-counter products, vitamins, herbals, and medical conditions. Share this list with your doctor, pharmacist, and nurse at each visit so that they can also screen for drug interactions. Also review the Medication Guide, prescription information, warning labels, and Drug Facts Label with each new prescription or OTC product. Labeling may change as new information is learned about medications, so it’s important to review the information frequently.


  1. Phillips KA, Veenstra DL, Oren E, Lee JK, Sadee W. Potential role of pharmacogenomics in reducing adverse drug reactions: A systematic review. JAMA 2001;286:2270-9.
  2. Institute for Safe Medical Practices:


E. William Iyamu, MD, PHDE. William Iyamu, MD, PHD, of DeArk Medical Center is a board-certified internal medicine doctor. He received his Medical Training at the University of Kansas School of Medicine, Kansas City, KS in 2008 and then completed his Internal Medicine Internship at the University of Nebraska Medical Center in Omaha, NE, followed by Internal Medicine Residency Training at Meharry Medical College, Nashville, TN. He then served as Chief Medical Resident at Meharry Medical College for one year and received additional training in Hematology/Oncology at the University of Arkansas for Medical Sciences, Little Rock, AR. He practiced as a Hospitalist for several years, including his position as a staff physician at The Carolinas Healthcare Systems. Dr. Iyamu has expertise in preventative health care and in the management of medically complex patient including, but not limited to, those with hypertension, heart disease, chronic lung disease, opioid used disorder and diabetes. His is married with children.

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