Drug Interactions: A Guide for Consumers


We live in a world where incredible drugs exist to treat many conditions that seemed untouchable in the past. According to data from the Centers for Disease Control and Prevention, an estimated 48.7 percent of Americans used at least one prescription in the last 30 days. Approximately 21.8 percent of Americans used three or more. It’s encouraging to know that there are options out there to address many of our common ailments but the impressive availability of medications also increases the possibility of drug interactions.

Drug interactions are combinations of medication with other substances that alter the medication’s effect on the body. They can cause your medications to be less or more potent than intended. They can also result in unexpected side effects, which may be harmful. If you use multiple medications, have certain health conditions, or are in the care of more than one doctor, you should be especially mindful of your medications. You also need to make sure that all of your doctors know all of the drugs, herbs, supplements, and vitamins you are using. Even if you take only one medication, it’s a good idea to talk with your doctor or pharmacist about what you are using, so you can identify possible interactions. This advice applies to both prescription and nonprescription drugs.

Types of Drug Interactions

  • Drug-drug: A reaction between two or more drugs. This can involve prescription medications, over-the-counter medicines (OTC), and herbs, vitamins, and supplements. An example of this is someone who takes a diuretic — a drug that attempts to rid the body of excess water and salt —and also takes ibuprofen. The ibuprofen may reduce the diuretic’s effectiveness because ibuprofen often causes the body to retain salt and fluid. 
  • Drug-food: When food or beverage intake alters a drug’s effect. If someone taking certain statins to lower cholesterol drinks a lot of grapefruit juice, this can cause too much of the drug to stay in the body. This may increase their risk for liver damage or kidney failure.
  • Drug-alcohol: Certain medications that should not be taken with alcohol. Often, combining the two can cause tiredness and delayed reactions, and can also increase your risk for negative side effects.
  • Drug-disease: The use of a drug that alters or worsens a condition or disease the person has. For example, certain decongestants people take for colds can increase blood pressure. This is a potentially dangerous interaction for people with high blood pressure (hypertension).
  • Drug-laboratory: When a medication interferes with a laboratory test. This can result in inaccurate test results. For instance, certain antidepressants (tricyclic antidepressants) have been shown to interfere with skin prick tests used to determine allergies someone may have.

Other Factors in Drug Interactions

It’s important to educate yourself on your potential for drug interactions but, at the same time, understand this information doesn’t tell you everything you need to know. Just because a drug interaction can occur doesn't mean that it will occur in every instance. Personal traits can play a role in whether a drug interaction will happen and whether it will be harmful. Specifics about your drugs, including dosage, formulation, and how you take them can also make a difference.

Personal medical history to consider:

  • Genetics: Variations in individual genetic makeup can make the same drug work differently in different bodies. As a result of their particular genetic code, some people process certain medications more quickly or more slowly than others, which may cause the drug levels to go down or go up more than expected. Your doctor will know which drugs require genetic testing to find the correct dose for you.
  • Weight: Some drugs are dosed according to your weight, while others are not. This could affect the dose, and could also increase or decrease the risk of drug interactions. If you have a substantial change in your weight, you could need a different dose of some medications.
  • Age: As we age, our bodies change in many ways, some of which may affect how our body responds to medications. The kidneys, liver and circulation system may slow down with age. This can slow the breakdown and removal of drugs from the body and may affect how long the drug is in the body.
  • Sex (male or female): Differences between the sexes, such as anatomy and hormones, can play a part in drug interactions. For example, the recommended dose of zolpidem (Ambien) given to women was lowered to half the amount prescribed to men. This happened after research found that women were more likely to have high levels of the drug in their system in the morning, when it could impair activities like driving.
  • 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 of that condition. Exercise can also change how medications work.  For example, people who use insulin to treat diabetes can experience hypoglycemia (low blood sugar) during exercise, and may need to adjust the time they eat and take their insulin to offset the drop in blood sugar.
  • 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 these, and may be higher or lower than the typical dose.  This is another reason why your doctor needs to know all the drugs you are taking before prescribing a new medication. 
  • How long you’ve been taking the drug: The body can become tolerant to some medications over time, or the drug itself may help the body to process it more quickly over time. The doses may have to be adjusted if they are taken for a long time. Two examples are pain drugs and anti-seizure drugs.

Drug factors to consider:

  • Dose: The dose is the amount of medication prescribed to be taken or administered in a given period of time. Two people taking the exact same drug may be prescribed different doses. Calculating the proper dose requires precision, and people should not alter their dosage without consulting with their doctor first.
  • How the drug is taken/administered: There are many different ways a drug can be administered. Some common ways we take drugs include orally (by mouth), rectally (by rectum), and topically (applied to the skin). The way medications enter the body can greatly alter the resulting effects.
  • Formulation: 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.
  • The order in which medications are taken:  Some drug interactions can be reduced or eliminated if the two drugs are taken at different times.  There are also drugs that may affect the absorption of other drugs.  Antacids like calcium tablets can prevent the absorption of the antifungal medication ketoconazole, for example. 

Reading Drug Labels

Speaking with your doctor or pharmacist is the best way to stay informed about your medications, but you should always read all drug labels and patient drug information you receive, whether the drug is prescription or OTC.  These will help you to better understand your drugs, and it may also prevent interactions.

Sections of an OTC drug label:

  • Active ingredient and Purpose: Lists the ingredients in the drug that serve therapeutic purposes. The “Purpose” section will say what each ingredient does (e.g. 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 will say 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 will be 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.
  • Inactive Ingredients: List of ingredients in the drug that do not serve a therapeutic purpose, such as colorings and flavorings.
  • Manufacturer contact information:  You can usually call the manufacturer on a toll free line if you have questions about the drug. Most companies staff these lines Monday through Friday.

Prescription drug labels

There are two kinds of prescription labels. The full label is actually the package insert, which is a long detailed document of the drug information and is usually found inside or attached to the prescription stock bottle. There is usually a summary version of this label that is attached, which is intended for patients.

The other kind of prescription label is more familiar to most people, and is attached to the individual bottle or package of medication that is dispensed directly to the patient.  This type of label contains your name, your doctor’s name, and the name of the drug, along with the strength, dose, directions and other identifying information.  This brief information is there to remind you about how to take the drug. The prescription vial may also have warning labels in the form of colorful stickers located directly on medication bottles. These have information about side effects and potential interactions.

Each new prescription includes detailed patient information about the use of the drug, which is written more clearly than most package inserts.  This information is often personalized with the patient’s name. 

The format and standards of both package inserts and prescription vial labels have been agreed upon and set by the United States Pharmacopeia (USP), a volunteer organization which sets national standards for pharmacy. Additionally, each state in the US has a board of pharmacy which further details all pharmacy procedures including labelling of prescriptions. 

To learn more about a prescription drug, ask for the package insert.  There is a summary section that is primarily intended for patients, and the complete current package insert that is primarily written for physicians and pharmacists. The package insert describes the drug and provides other details, including:

  • How the drug works, and information about clinical trials for the drug
  • How to take the drug and any precautions (ex: 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

Learning more about Drug Interactions

Talk to your doctor or pharmacist in order to get the most accurate and up-to-date information about your personal risk of drug interactions. Make sure they know all of the medications you are 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:

  1. How does this drug work? What effect may it have in my body?
  2. Can I take this medication with other prescription drugs, OTC drugs, herbs, vitamins or supplements?
  3. Are the any foods or beverages I should avoid?
  4. Should I avoid alcohol while taking this drug?
  5. What are signs of a drug interaction?
  6. What should I do if I experience a drug interaction?
  7. Where can I get additional information about this drug?
  8. Can I take this drug while I’m pregnant or breastfeeding? (If applicable)

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

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