Most medications produce several different effects in your body. They produce the desired effect of relieving symptoms or helping to cure an illness or medical condition, but they can also produce other effects that you don't want. Although most people use the term "side effect," the term adverse drug reaction is a more appropriate general term for effects that are undesired, unpleasant or potentially harmful.

In general, adverse reactions to drugs are fairly common. Almost any medication can cause an adverse reaction. Reactions range from irritating or mild side effects, such as nausea and mild rash, to allergic responses including life-threatening anaphylaxis (see "Types of drug allergies" in this feature).

It is estimated that preventable adverse drug reactions may make up as much as 80% of all adverse outcomes leading to emergency visits and hospital admissions, causing a disproportionate healthcare cost increase.

What is the difference between a side effect and a drug allergy? Sometimes it's tough to know the difference! But having a side effect to a medication does not mean you're allergic to it. Side effects are more common, do not involve the body's immune system, are more likely to occur at higher doses, and can often be prevented. Different people may have different side effects from the same medication. Researchers are finding out that genetics may play a role in how people respond to medication.

Side effects are usually an extension of the way the medication works in your body. For example, a pill taken to lower blood pressure may lower it too much. This could make someone feel faint when they stand up. In many cases, the higher the dose of the medication, the more likely a side effect will occur.

Drug allergies are less common, involve the body's immune system, can occur with any dose of medication, and can be life threatening.