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Answer:Not only can a drug interaction cause unexpected side effects, it can make a medication less effective or it can increase the action of a particular drug. Drug interactions can range from mild to life threatening.

There are four main kinds of drug interactions.

A drug/drug interaction happens when two or more different drugs react with each other and cause an unwanted side effect or change how the drug acts in the body. An example is when a person takes both a sleep medication (a sedative) and an antihistamine for allergy relief. Both these medications may cause drowsiness; together, they can seriously slow a person's reactions and make it very dangerous to drive a car.

The caffeine (which is considered to be a drug) in everyday foods—such as coffee and chocolate—also can interact with certain other drugs. And the nicotine in tobacco products—again, also a drug—can interact with other medications, especially nicotine-replacement products.

A drug/food interaction can occur when certain foods or beverages are consumed with certain medications. An example is certain statin medications used to treat high cholesterol and grapefruit. Both grapefruit and its juice have been found to interfere with the way some of these medications are processed in the body, causing potential adverse effects.

A drug/condition interaction may result when a person's existing medical condition has the potential to make certain medications potentially harmful. You see warnings about this all the time on over-the-counter remedies, for example: "Do not take this medication if you have high blood pressure." Taking the medication could make the condition worse.

A drug/alcohol interaction is actually a drug/drug interaction, since alcohol itself is a drug. Drinking alcohol while taking certain medications can cause an adverse effect, such as causing extreme drowsiness or slowing a person's reactions.

Be sure to always read the labels on your prescription and over-the-counter medications, and ask your pharmacist if you need additional information about your medications.

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DISCLAIMER: The information provided is not a substitute for medical advice. Consult your physician for diagnosis and treatment of your medical condition. Advances in medicine may cause this information to become outdated, invalid or subject to debate. Professional opinions and interpretations of the scientific literature may vary. Walgreens' terms of use and general warranty disclaimer apply to all services provided. If you are in need of immediate medical attention, contact your physician, poison control center or emergency medical professional. If you need to speak with a pharmacist for non-emergency matters, contact your local Walgreens pharmacist or call a Walgreens.com pharmacist toll-free at 1-877-250-5823.