A drug-drug interaction results when two or more medicines interact with each other in a way that prevents the drug from working as expected. The interaction can increase or decrease the effectiveness of the medicines, cause unexpected side effects, or increase the risk of known side effects. While some interactions may be helpful or beneficial, most are unwanted. Some drug interactions can be very harmful.

The likelihood of drug interactions rises as the number of drugs being taken rises. These medicines may be prescription and/or over-the-counter (OTC) medications, including vitamins, dietary supplements, and herbal medicines. People who take numerous medications are at the highest risk for a drug-drug interaction.

A drug-food interaction results when a medicine reacts differently if taken with a food and/or beverage. For example, foods may decrease the absorption of a medicine into the blood making the drug less effective. Or, mixing some medicines with alcohol can cause extra drowsiness or liver damage. See the table below for a few examples.

Some medicines can cause unwanted and even harmful effects if you have certain medical conditions (e.g., diagnosis, contraindication, allergy). For example, taking a medicine to which you have an allergy can result in severe breathing and skin reactions. Or, you may worsen your condition (e.g., high blood pressure) by taking an over-the-counter medicine (e.g., decongestant such as pseudoephedrine). Keep in mind that a medicine's inactive ingredients (e.g., soy, gluten) can also cause reactions with existing medical conditions.

1. Know what is in each medicine you take. To do this, carefully read the labels of all prescription and over-the-counter (OTC) medicines to learn the active and inactive ingredients they contain.
2. Know the benefits and risks of the medicines you take. For OTC medicines, be sure to read the "Warnings" and "Other Information" sections of the Drug Facts label.
3. Ask your doctor or pharmacist if it is safe to take a medicine with your condition(s) or with other medicines, vitamins, or herbal products you take.
4. Keep an up-to-date list of all the prescription and OTC medicines, vitamins, and herbal products you take. Bring the list with you anytime you visit your doctor's office, pharmacy, or hospital. You can access a form to keep track of your medicines here.
5. If possible, purchase all of your prescription and OTC medicines from one pharmacy.