Adverse Reaction to a Medication or Drug

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An adverse reaction is an unwanted or unexpected negative reaction to a medication or treatment that is used in an approved manner.

Adverse Reaction vs. Side Effects

It can be confusing when you see the terms adverse reactions and side effects. Do they mean the same thing? Sometimes, but not always. Side effects can mean symptoms that are either expected or not expected related to the intake of a medication. In many cases, the term "adverse reaction" is used in the same way. In others, the term adverse reaction refers to symptoms that were not expected


There are many terms that you may read alongside a list of adverse reactions or side effects of a drug. A few of these include:

  • Dose-dependent — A dose-dependent side effect is one that is expected, and increases along with an increase in the dose of a drug. An example would be a medication which causes fatigue, in which tiredness worsens sequentially with increasing doses of the drug. In contrast, some adverse reactions may not vary depending upon the dose of a drug.
  • Idiosyncratic — An idiosyncratic reaction is one that is both undesirable and not expected. An example would be a medication which causes diarrhea for a patient when it ordinarily would cause constipation.

Adverse Reaction vs. Allergy

It's important to make a note about allergies when talking about drug reactions. Generically the terms adverse reaction or side effect could refer to both allergies or non-allergies. An allergic drug reaction or adverse reaction refers to a reaction in which your body recognizes the drug as foreign, and tries to "fight it off."

True allergic reactions to drugs are responsible for only 1 out of 10 adverse drug reactions.

Symptoms and Severity

Adverse reactions can be very mild or even undetectable except through lab testing, or they can be serious and life-threatening.

Timing of Drug Reactions

Adverse reactions may occur at any time after starting a drug, including those due to allergies. This is a confusing point to many people, who may dismiss their medication as a cause for newly acquired symptoms if they begin some period of time after starting a medication.

Types of Reactions

You may think of a rash when talking about adverse reactions, but these reactions can occur in any organ of a system of the body.

Adverse Reactions Summary

As a summary of some of the confusing terms you may hear about drug reactions:

  • Adverse reactions are undesirable.
  • Adverse reactions can be expected or unexpected. On the extreme side of unexpected would be idiosyncratic in which the opposite reactions would be expected.
  • They can be due to an allergy to the drug or non-allergic processes.
  • The can be mild or instead life-threatening.
  • Adverse reactions can be variable, they can be different for everyone.
  • Some adverse reactions can be due to the interaction of a drug with another drug, rather than a reaction of the body to the drug itself.


Adverse reactions can be mild, such as a simple rash, or severe and life-threatening in nature. They can occur immediately when a treatment is started, or develop over time. Some adverse reactions are common and can be anticipated by your doctor, whereas others occur very rarely. Some symptoms that may occur as an adverse reaction can include:

  • Gastrointestinal bleeding
  • Heartburn
  • Fatigue/sleepiness
  • Nausea and diarrhea
  • Lightheadedness or dizziness 
  • Diarrhea or constipation
  • Skin rashes

Adverse Reactions Aren't Always Bad

It's important to note that sometimes a medication should be continued even if someone has an adverse reaction. An example would be the medication Tarceva used for lung cancer. This drug often causes an acne type of rash. In this case, however, it appears that having a rash, and the more severe a rash is, the better the drug is working to fight lung cancer. In these situations, you and your doctor will need to weigh the benefit of the treatment against the side effect of the drug.

If You Suspect an Adverse Reaction

If you suspect you are having an adverse reaction to a medication or treatment, it is important to let your doctor know as soon as possible. Also, check out common drug reactions and interactions, and common arthritis medication side effects.

Depending on the severity, call 911, or make an appointment with your doctor

Bring all medications, including any vitamins, over-the-counter medications, and nutritional supplements to your appointment.

Examples: Evelyn developed hives when she began the new medication to help with her lung cancer. Her oncologist told her this was an adverse reaction that some people have, and recommended a different medication.

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Article Sources

  • American Academy of Allergy Asthma and Immunology. Medications and Drug Allergic Reactions.