We live in a world where drugs exist to treat many conditions that seemed untouchable in the past.

In a report that looked at U.S. prescription drug use from 2015 to 2018, the Centers for Disease Control and Prevention (CDC) found that an estimated 48.6% of Americans used at least one prescription in the past 30 days.

It’s encouraging to know that there are options to address many of our common ailments. But the increased availability of medications also increases the possibility of drug interactions.

Drug interactions involve combinations of a medication with other substances that alter the medication’s effect on the body. This can cause the medication to be less or more potent than intended or result in unexpected side effects.

If you use multiple medications, have certain health conditions, or see more than one doctor, you should be especially mindful of your medications. You also need to make sure that each of your doctors are aware of all of the drugs, herbs, supplements, and vitamins you’re using.

Even if you take only one medication, it’s a good idea to talk with your doctor or pharmacist about what you’re taking to identify possible interactions. This advice applies to both prescription and nonprescription drugs.

There are several different types of drug interactions to be aware of. Let’s explore each one a little further.


A drug-drug reaction is when there’s an interaction between two or more prescription drugs.

One example is the interaction between warfarin (Coumadin), an anticoagulant (blood thinner), and fluconazole (Diflucan), an antifungal medication. Taking these two drugs together can lead to a potentially dangerous increase in bleeding.

Drug-nonprescription treatment

This is a reaction between a drug and a nonprescription treatment or two nonprescription treatments. These include over-the-counter (OTC) medications, herbs, vitamins, or supplements.

An example of this type of interaction can occur between a diuretic — a drug that attempts to rid the body of excess water and salt — and ibuprofen (Advil). The ibuprofen may reduce the diuretic’s effectiveness because ibuprofen often causes the body to retain salt and fluid. There are both prescription and nonprescription diuretics.


This happens when food or beverage intake alters a drug’s effect.

For example, some statins (used to treat high cholesterol) can interact with grapefruit juice. If a person who takes one of these statins drinks a lot of grapefruit juice, too much of the drug may stay in their body, increasing their risk of liver damage or kidney failure.

Another potential outcome of the statin-grapefruit juice interaction is rhabdomyolysis. This is when skeletal muscle breaks down, releasing a protein called myoglobin into the blood. Myoglobin can go on to damage the kidneys.


Certain medications are not suitable for use with alcohol. Often, combining these drugs with alcohol can cause tiredness and delayed reactions. It can also increase your risk of negative side effects.

For example, simultaneous consumption of alcohol, or alcohol-containing medications, with metronidazole can cause flushing, vomiting, and abdominal pain. Metronidazole is a common antibiotic.


This interaction is when the use of a drug alters or worsens a condition or disease. Additionally, some medical conditions can increase the risk of side effects from specific drugs.

For example, some decongestants that people take for colds can increase blood pressure and are not suitable for people with high blood pressure (hypertension).

Another example is metformin (a diabetes drug) and kidney disease. People with kidney disease should use a lower dosage of metformin or not take it at all. This is because metformin can accumulate in the kidneys of people with this disease, increasing the risk of severe side effects.


Some medications can interfere with specific laboratory tests. This can result in inaccurate test results.

For instance, tricyclic antidepressants have been shown to interfere with skin prick tests used to determine whether someone has certain allergies.

While it’s important to educate yourself on your potential for experiencing drug interactions, understand that this information doesn’t tell you everything you need to know. Just because a drug interaction can occur doesn’t mean it will.

Personal traits can play a role in whether a drug interaction will happen and if it will be harmful.


Variations in individual genetic makeup can make the same drug work differently in different bodies.

Some people — because of their specific genetic code — process certain medications more quickly or more slowly than others. This may cause the drug levels to go down or go up more than expected. Your doctor knows which drugs require genetic testing to find the correct dosage for you.


Some drugs are dosed according to how much a person weighs.

Weight changes could affect dosage and also increase or decrease the risk of drug interactions. So if you have a substantial change in your weight, you may need a different dosage of some medications.

For example, doctors prescribe varying strengths of low weight heparin, such as enoxaparin, and antibiotics, such as vancomycin, depending on a person’s body weight.


As we age, our bodies change in many ways, some of which may affect how we respond to medications. The kidneys, liver, and circulation system may slow down with age. This can slow the breakdown and removal of drugs from our bodies.


Differences between the sexes, such as anatomy and hormones, can play a part in drug interactions.

For example, studies show that men metabolize zolpidem (Ambien) at double the rate of women.

Lifestyle (diet and exercise)

Certain diets can be problematic when combined with medication.

For example, research has shown that high fat intake can reduce the response of bronchodilators, which people with asthma use to treat symptoms.

Exercise can also change how medications work.

For example, people who take insulin to treat diabetes can experience hypoglycemia (low blood sugar) during exercise. So they may need to adjust the time they eat and take their insulin to offset the drop in blood sugar.

Smoking cigarettes can also affect the metabolism of some drugs. Be sure to tell your doctor that you smoke if they’re recommending you start a new medication.

If you’re thinking about quitting smoking, your doctor can help you create a personal plan with actionable steps to help you stop.

High levels of vitamin K in the diet can also inhibit the blood thinning medication warfarin.

How long the drug is in your body

Many factors affect the speed at which the body absorbs and processes drugs. The right dose for each person may depend on such factors and may be higher or lower than the typical dose.

This is another reason why it’s important to tell your doctor about all the drugs you’re taking when they’re considering prescribing a new medication.

How long you’ve been taking the drug

The body can become tolerant to some medications, or the drugs themselves may help the body process them more quickly over time. So dosages may need to be adjusted if they’re taken for a long time. Two examples are pain medications and antiseizure drugs.


The term “dose” is the amount of medication prescribed to be taken or administered. (You may sometimes hear the term “dosage,” which refers to an amount of medication given at specific periods of time — for example, once a day.)

Two people taking the exact same drug may be prescribed different doses. Calculating the proper dose requires precision, so you should not alter how much of a medication you take without consulting with your doctor first.

How the drug is taken or administered

There are many different drug administration methods. Some common ways drugs are taken include orally (by mouth), by injection, and topically (applied to the skin). The way medications enter the body can greatly alter the resulting effects.


The formulation of a medication is the specific mixture of ingredients the drug contains. A medication’s formulation is important because it can determine, in part, how the drug acts in the body as well as its effectiveness.

For example, doctors prescribe intravenous vancomycin to treat certain gram-positive bacterial infections, whereas oral vancomycin is for the treatment of diarrhea caused by Clostridium difficile bacteria.

The order in which medications are taken

Taking drugs at different times can reduce the risk of adverse interactions.

Certain drugs may affect the absorption of other drugs when taken one before the other. Antacids like calcium tablets can prevent the absorption of the antifungal medication ketoconazole, for example.

Speaking with your doctor or pharmacist is the best way to stay informed about your medications.

But always reading all drug labels and patient drug information you receive is also necessary, whether the drug is prescription or OTC. Doing this can help you better understand your medications, and it may also prevent interactions.

OTC drug labels

OTC drug labels typically include the following information:

  • Active ingredient and purpose: Lists the ingredients in the drug that serve therapeutic purposes. The “Purpose” section explains what each ingredient does (for example, nasal decongestant, antihistamine, pain reliever, fever reducer).
  • Uses: A short description of what symptoms or conditions the drug is meant to treat.
  • Warnings: The section that provides important information about using the drug safely. It explains when to stop or not use the drug and when to consult with a doctor about its use. Side effects and potential interactions are also listed here.
  • Directions: Instructions for how much of the medication should be taken and how often. If there are any special instructions for how to take the drug, they’re listed here.
  • Other information: This section often has information about how to properly store the drug. It may also give additional information about certain ingredients the drug contains, such as the amount of calcium, potassium, or sodium. These details can be important for people with allergies or dietary restrictions.
  • Expiration date: Date up to which the manufacturer guarantees the drug’s safety and effectiveness.
  • Inactive ingredients: List of ingredients in the drug that do not have a therapeutic purpose, such as colorings and flavorings.
  • Manufacturer contact information: You can usually call the manufacturer via a toll-free number if you have questions about the drug. Most companies staff these call centers Monday through Friday.

Prescription drug labels

There are two kinds of prescription labels — package inserts and patient package inserts (PPI). The Food and Drug Administration (FDA) regulates the format and standards of both types of labels.

You may also see a package insert called the prescribing information. It’s a detailed document containing information about the drug and is usually found inside or attached to the prescription stock bottle.

To learn more about a prescription drug, ask for the package insert. The package insert describes:

  • how the drug works and information about clinical trials for the drug
  • how to take the drug and any precautions (such as whether it should be taken with food)
  • what conditions the drug is used to treat
  • warnings about potential side effects or adverse reactions
  • possible interactions with other drugs, supplements, foods, or beverages
  • dosage information and instructions on what to do in case of an overdose
  • other information, such as what the drug looks like and how to store it

The prescription stock bottle may also have warning labels in the form of colorful stickers located directly on bottles. These have information about side effects and potential interactions.

The PPI is more familiar to most people. It’s the information that’s given with the medication that’s dispensed directly to you. The PPI includes detailed information about the use of the drug, which is written more clearly than most package inserts.

Additionally, your prescription label should contain your name, your doctor’s name, and the name of the drug, along with the strength, dose, directions, expiration date, and other identifying information. This brief information is there to remind you about how to take the drug.

Talk with your doctor or pharmacist to get the most accurate and up-to-date information about your personal risk of drug interactions. Make sure they know all the medications you’re taking.

Have a clear conversation about potential foods, OTC drugs, and diseases that could cause problems when combined with your medications.

Some questions to ask:

  • How exactly does this drug work in my body? What potential side effects could I experience?
  • Can I take this medication with my other prescriptions? If so, should I take it at a different time than my other medications?
  • I also take the following OTC drugs, herbs, vitamins, or supplements. Is this drug safe to take with them?
  • Are there any specific foods or beverages that I should avoid when I’m taking this drug? If so, why?
  • What potential effect could alcohol consumption have while taking this drug?
  • Can you also explain the signs of a drug interaction that I should look out for?
  • What should I do if I experience severe side effects or a drug interaction?
  • I’d like more information about this drug. Can you provide me with a copy of the package insert? If not, where can I find it online?
  • (If applicable) Can I take this drug while I’m pregnant or breastfeeding?
  • Can this drug be crushed or chewed if I find it hard to swallow, or mixed with food or drink to mask its taste?

If you have any concerns or questions about medications you’re taking or planning to take, consult your doctor. In particular, people who are pregnant or breastfeeding should check with their doctor before taking any new medications.