Drug Interactions

There are thousands of prescription drugs, with more and more medications introduced every year, and hundreds of dietary supplements and herbal products available in pharmacies and stores in the U.S. Consumers should be aware of the dangers of combining certain drugs and other substances, including alcohol.

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Patients who take prescription medicine should know that in addition to potential side effects, it’s important to be aware of how the drugs interact with other drugs, with supplements and certain foods.

According to the government’s National Health Survey, about 20 percent of adults are taking three or more drugs. An expert with the U.S. Food and Drug Administration thinks many more are taking two or more prescriptions.

Sometimes a doctor will prescribe a combination of drugs with the intent of creating beneficial interactions that enhance the effects of medication. However, often, the consequences of interactions are unintended and can be dangerous, potentially fatal.

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Multiple Medications and Supplements

A 2016 study in JAMA Internal Medicine found that older adults are increasingly using multiple medications and supplements. The study found 36 percent regularly using five or more drugs or supplements and 15 percent at risk for potential major drug interactions.

Particularly dangerous reactions can occur when a person takes two or more drugs that have similar properties because the effects are magnified. Also dangerous is a situation in which the person ingests drugs with opposite properties that cancel the medicinal benefit of both drugs. Another common adverse interaction can occur when one drug alters the concentration of a substance that is normally present in the body.

Interaction Risks

The more medications a patient takes, the higher the risk that the drugs will interact with each other.

The drug-interaction risks are:

The drug-interaction risk for two medications is 15 percent.

15 percent
for two medications

The drug-interaction risk for five medications is 40 percent.

40 percent
for five medications

The drug-interaction risk for seven medications is 80 percent.

80 percent
for seven medications

This is a particular concern for older people, as more than half take five or more medications, and 12 percent take 10 or more.

Risks Vary

For people in hospitals, adverse drug interactions may be the fourth leading cause of death.

These combinations can affect how prescriptions work in the body. They can enhance or decrease the effects of the drugs. They can also contribute to the possibility of dangerous side effects. The results vary depending on the person and other things like the particular combinations, the doses, the timing and even genetics.

In some instances, drug interactions discovered after medication approval were so dangerous that the drugs were pulled from the market.

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Drug Interaction Health Risks

Possible side effects from these combinations also vary widely.

They include:

  • Drowsiness
  • Rapid heartbeat
  • Changes in blood pressure
  • Bleeding
  • Flushing
  • Diarrhea
  • Ruptured Achilles tendon
  • Coma
  • Death

Interaction Factors

Drug interactions are complicated and largely hard to predict. Not everyone who takes drugs that are known to interact with each other will experience the effects.

The likelihood of interactions causing problems can depend on several factors including:

  • Genetics
  • Age
  • Lifestyle, such as the patient’s diet and her exercise habits
  • Other medical conditions
  • The length of time both drugs are taken.

It’s best to talk to your doctor or pharmacist about the potential effects of combining medications, as well as dietary supplements and even certain foods.

Drug Interaction Types

There are different types of drug interactions. These are:

  • Drug-drug interactions - Drug-drug interactions happen when drugs react with each other. One example of this is when a patient takes a sedative to fall asleep and also has allergies and takes an antihistamine. Combining both of those drugs can slow the patient’s reactions and make it dangerous to drive or operate machinery.
  • Drug-food/beverage interactions - Drug-food/beverage interactions happen when drugs react to what you eat or drink. Alcohol is a well-known example of this. Drinking an alcoholic beverage while taking some medications can also slow your reactions and make you sleepy.
  • Drug-condition interactions - Drug-condition interactions happen when you have a medical condition that reacts badly to certain drugs. High blood pressure may affect nasal decongestants, for example. And if you are allergic to certain substances, you may not be able to take some drugs. This is also true for conditions that affect the ability of the kidneys or the liver to function, as well as genetic conditions that affect drug metabolism.
  • Drug-supplement interactions - Drug-supplement interactions are when your medication reacts to herbal or dietary supplements.
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Drug-Drug Interactions

A drug-drug interaction typically involves what’s known as a precipitant drug and an object drug. The object drug is the one with a therapeutic affect that is changed by the drug interaction. The precipitant drug is the one that affects the pharmacologic action of the object drug.

Common precipitant drugs — drugs that have an effect on the action of other drugs — include nonsteroidal anti-inflammatory drugs and certain antibiotics. Common object drugs include warfarin, fluoroquinolones, antiepileptic drugs and oral contraceptives.

Some drugs act both as object drugs and precipitants.

The consequences of drug interactions are as diverse as the possible drug combinations. Nonsteroidal anti-inflammatory drugs taken at the same time as corticosteroids, anticoagulants or antiplatelets can increase the risk of gastrointestinal bleeding. Fluoroquinolones taken at the same time as corticosteroids can lead to increased risk of rupture of the Achilles tendon.

Some common drugs and examples of how they interact with other drugs, supplements and food:


The most widely prescribed class of antidepressants today is selective serotonin reuptake inhibitors (SSRIs). They work to elevate the serotonin levels in the brain, which is thought to ease the symptoms of depression.

One serious complication, serotonin syndrome, can occur when an SSRI interacts with other drugs that contain the same ingredients, especially pain medications. The onset of serotonin syndrome can occur within minutes and can be deadly if not treated immediately.

St. John’s wort, a dietary supplement used to treat depression, can cause serotonin syndrome when it is combined with an SSRI. SSRIs also are known to interact with over-the-counter antihistamines, causing extreme drowsiness.

Common SSRIs include:









There are no proven interactions with food when a patient takes an SSRI. At least one study, however, suggests that eating kiwifruit, which has high natural levels of serotonin, can boost the effectiveness of SSRIs. Also, certain SSRIs can cause an upset stomach or heartburn if they are taken on an empty stomach.

Blood Pressure Medication

Some of the most common medications used to treat high blood pressure, or hypertension, are diuretics known as hydrochlorothiazides (HCTZ). HCTZ drugs should not be taken with heart-rhythm medications because the combination can lower potassium levels to the point that fatal cardiac arrhythmia can occur. OTC decongestants also should not be used when a patient is taking HCTZ drugs because they decrease the effectiveness of the blood-pressure medication.


Eating black licorice can have a negative effect on certain blood pressure prescriptions.

One food interaction has been documented for HCTZ drugs. Glycyrrhizin — found in black licorice, some sweeteners and herbal teas — can have a negative effect on potassium levels, which can damage the heart.

Proton Pump Inhibitors

Taking proton pump inhibitors, which reduce gastric acid, along with antibiotics can increase the risk of irregular heartbeat, which can be life threatening. A 2016 study found that taking the heartburn drug Prevacid (lansoprazole) at the same time as ceftriaxone, an antibiotic, can make it more likely that the patient will develop a dangerous, life-threatening condition called long QT syndrome, which can be fatal.

When proton pump inhibitors are taken with antibiotics, there is a risk of clostridium difficile infection, a bacterium with symptoms from diarrhea to colon inflammation that can be life threatening.


Within classes of drugs, some pose a higher risk of interaction than others. For example, different statins, which are taken for cholesterol, can react differently to other drugs.

Drugs that can interact dangerously with different statins include gemfibrozil, cyclosporine, clarithromycin, azole antifungals, protease inhibitors and colchicine. Drinking alcohol while taking statins can increase the risk of liver damage.

Muscle and Kidney Damage

One of the most common side effects of statins is muscle soreness. Mixing statins with certain drugs, dietary supplements or foods can damage muscles and even lead to kidney failure. For this reason, patients are advised not to mix statins with prescription oral fungal (yeast infection) medication or vitamin B complex.


Warfarin, a blood thinner, interacts with a number of other drugs and supplements. Some other medications increase the body’s metabolism of warfarin and the risk of thrombosis, or the formation of blood clots.

The drugs include these antiepileptic medications:

  • Phenytoin
  • Carbamazepine
  • Rifampin
  • Cholestyramine
  • Phenobarbital

On the opposite end of the spectrum, some drugs decrease the metabolism of warfarin.

These drugs include:

  • Fluoroquinolones
  • TMP/SMX, which are antibiotics
  • Macrolides

Other common drugs that interact with warfarin include simvastatin, acetaminophen, prednisone and omeprazole. Supplements also interact with warfarin, sometimes increasing the risk of bleeding. These include omega-3 fatty acids, garlic, gingko and saw palmetto.

Leafy green vegetables and other vegetables high in vitamin K can affect warfarin’s ability to prevent clots. Cranberry juice and other cranberry products can also affect how warfarin works.

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Food-Drug Interactions

Certain foods can have an effect on the medication you take. This can prevent a drug from working the way it should, increase or decrease a side effect of the drug or cause a new side effect. Medicine can also affect how your body digests and processes food.

Some examples of different food and beverages and how they can affect certain drugs:


According to the National Institutes of Health (NIH), about 71 percent of adults in the United States drink alcohol. Alcoholic beverages can interact with a wide range of medications, including many that are commonly prescribed, and you should consult with your doctor or pharmacist about whether you can drink while taking your prescription drugs.

According to NIH research, 42 percent of adults who drink also use medications that are known to interact with alcohol. This is true for nearly 78 percent of people over 65.

For example, alcohol can add to side effects of some drugs. It can add to the drowsiness caused by antihistamines and it can cause liver damage if you take acetaminophen or other pain relievers and fever reducing medicines. The chance for severe liver damage increases if you drink three or more alcoholic drinks a day. Alcohol can also increase the chances of dangerous side effects with narcotic medications. These risks include coma and death.

Statins and Nitrates

Avoid alcohol if you are taking statins because it increases the chance of liver damage. Drinking alcohol while taking nitrates, which are given for angina, may add to the blood-vessel-relaxing effect and dangerously lower blood pressure.

Refrain from drinking alcohol if you’re using theophylline medicines given for asthma. The two taken together can increase the chance of nausea, vomiting, headache and irritability.

Antibacterial and Psychiatric Drugs

Drinking alcohol while taking the antibacterial metronidazole or up to a day after finishing the medication can cause nausea, stomach cramps, vomiting, flushing and headaches.

The antifungal griseofulvin can make the effects of alcohol worse. Together, they can cause a fast heartbeat and flushing.

Alcohol can also add to the side effects, like drowsiness, of certain psychiatric medications, including antipsychotics, antidepressants and sleep medications.


Ingesting caffeine when using bronchodilators can increase the chance of side effects such as excitability, nervousness and rapid heartbeat.

Grapefruit Juice

Grapefruit juice contains compounds known as furanocoumarins, which change the characteristics of some drugs. These compounds are not present in other citrus juices. Consequently, grapefruit juice can affect how the body metabolizes drugs, changing the level of the medication in the blood.

These drugs include:

  • Antihistamines
  • Blood pressure medications,
  • Thyroid replacement drugs
  • Birth control
  • Antacids
  • Dextromethorphan (cough suppressant)

If you drink more than a quart of grapefruit juice a day, it can increase the levels of certain statins, but not all statins, in your body and raise the chance of side effects.

Leafy Green Vegetables


Although nutritionists recommend including leafy green vegetables in a healthy diet, you should be aware that they can interact with some prescription drugs.

Leafy green vegetables are high in vitamin K and can lower the ability of blood thinners to prevent clots. Some blood thinners like warfarin work by blocking clotting factors that are dependent on vitamin K.

Foods high in vitamin K include broccoli, cabbage, collard greens, spinach, kale, turnip greens, and brussel sprouts.

Leafy vegetables also are high in potassium, as are bananas, oranges and salt substitutes. Taking ACE inhibitor and certain diuretics can increase the amount of potassium in your body. Too much potassium can harm you, leading to irregular heartbeat and rapid heartbeats, also known as palpitations.

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Drug-Supplement Interactions

Americans spend more than $28 billion each year on dietary supplements. These include vitamins, minerals, herbal and other substances that are purchased over the counter at drug stores, grocery stores and health nutrition stores.

According to a survey of people aged 50 and over by the American Association of Retired Persons, 59 percent of respondents had used supplements the previous month, and 52 percent took them almost daily.

A study published in JAMA in 2016, found that older adults are using more and more supplements. Researchers found an increase of almost 50 percent increase in the use of multiple supplements, despite, the author wrote, “no evidence of any clinical benefits.”

Some of these supplements can change how the body absorbs, metabolizes or excretes drugs and have an effect on how potent the drug is in the system. But research on herb-drug interactions has not been rigorous, and information is mostly inferred from sources such as animal studies and other indirect means.

Still, experts have compiled information about some common supplements and their effect on different prescription medications. Here are a few examples:


Calcium supplements may reduce the effectiveness of some drugs, including antibiotics, such as quinolones and tetracylines, calcium channel blockers, osteoporosis drugs and thyroid drugs. It may increase the risk of kidney damage by increasing calcium blood levels when taken with antihypertensives including thiazide diuretics and it may increase the effects of digoxin.

Fish Oil

Taking fish oil may reduce the effectiveness of some cancer drugs and may increase the effects of blood pressure medications and blood thinners.


Garlic is sometimes taken in the belief that it can treat high cholesterol, heart disease and high blood pressure. The high doses of garlic contained in supplements can act as a blood thinner. This can be dangerous when taking blood-thinning medications and increase the risk of excessive bleeding. There is some evidence that garlic may affect the body’s metabolism of the HIV antiviral drug saquinavir, but that risk appears to be low.

Taking garlic in supplement form may also reduce the effectiveness of birth control pills, HIV/AIDS drugs, cyclosporine and isoniazid for tuberculosis.

Ginkgo Biloba


Combining the supplement gingko biloba with some prescription drugs may increase the risk of internal bleeding or stroke.

This herbal supplement can also thin the blood. Taking it with other substances that have that effect, such as aspirin, vitamin E and warfarin, may increase the risk for internal bleeding or stroke. Ginkgo and ginseng can also have dangerous interactions with certain psychiatric medications, including those given for anxiety. It can reduce the effectiveness of drugs including anticonvulsants, diabetes drugs and HIF/AIDS drugs.

St. John’s Wort

This herbal supplement can make certain drugs less effective. These drugs include medications for HIV/AIDS, heart disease, depression, organ transplants and birth control.

If you are taking digoxin, avoid combining it with senna and St. John’s wort, which may decrease the amount and action of digoxin in your body.

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How to Avoid Drug Interactions

When you get a new prescription, you should ask your doctor or pharmacist some questions:

  1. Can I take it with other drugs and supplements?
  2. Shoold I avoid certain food or drinks?
  3. What signs of possible interactions should I be aware of?
  4. How does the drug work in my body?
  5. Where can I get more information about this drug or my condition?

Drug Labels

You should also make sure your doctor and pharmacist know about vitamins and supplements you take, and read the information provided by the pharmacy with your prescription. Carefully read the drug interaction precaution information.


One way to avoid dangerous drug interactions is to fill your prescriptions at the same pharmacy.

It’s best if you can obtain all your prescriptions from single pharmacy, which can keep track and flag possible interaction issues.

Medication List

It is extremely important to keep a complete list of all your medications and bring that list to every medical appointment to help your doctor check for possible drug interactions. It’s probably wise to carry the list with you at all times. In addition to the name of the medication, you should include the size of the dose and how often you take the drug. You should also ask your doctor and your pharmacist whether there is a chance of drug interactions.

Online Tools

The U.S. Food and Drug Administration has a tool you can find here to help you create a record to keep track of your medicines and dietary supplements.

The AARP also has an interaction checker where you can enter the name of a drug or pick from a list of common drugs and it will give you possible interactions to be alert for.

Avoiding Interactions

Your doctor or pharmacist can take steps to address possible drug interactions.

These steps may include:

  • Avoiding certain drug combinations because they are too risky.
  • Adjusting the dose of the object drug.
  • Spacing the times between taking drugs that interact with each other. For example, certain object drugs may need to be taken at least two hours before or four hours after the precipitant drug, allowing the object drug to be absorbed by the body before the precipitant drug is introduced.
  • Monitoring for side effects through laboratory tests or close observation, adjusting dosages as necessary.


Elaine Silvestrini is a career journalist with a strong desire to learn, explain, and help people. At Drugwatch, Elaine has reported about trials over whether talcum powder caused ovarian cancer and allegations that Androgel causes heart problems in patients who use it. She has chronicled the billions of dollars generated for big pharmaceutical companies by certain drugs and efforts to warn consumers about the dangers of specific medical problems. Elaine has received six health literacy certificates for completing courses offered by the Centers for Disease Control and Prevention. She brings more than 20 years of experience covering state and federal court systems, learning the intricacies of criminal and civil law, developing investigative pieces about how the law affects people’s lives and digging through and digesting reams of court records on a daily basis.

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