Adverse Reaction to a Medication or Drug

An adverse reaction is an unexpected negative reaction to a medication or treatment that is used in an approved manner. While sometimes used interchangeably with side effects, the term side effect often refers to effects that are minor or confer less harm. Adverse drugs reactions may occur shortly after a medication is used, or may not be seen for decades. As a leading cause of illness and death in the United States, the importance cannot be overstated. Any prescription or over-the-counter drug, as well as nutritional supplements, has the potential to cause adverse reactions.

woman with skin rash on arm
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An adverse drug reaction is defined as an unintended effect of a medication that is harmful or unpleasant.

Adverse Drug Effects vs. Adverse Drug Reactions

The terms "adverse drug effects" and "adverse drug reactions" are often used interchangeably, but there are some differences. An adverse drug effect refers to an effect that may be seen on lab tests or imaging studies, whereas an adverse drug reaction usually refers to clinical signs and symptoms. Adverse drug effects may or may not lead to adverse drug reactions, but adverse reactions may occur without preceding adverse drug effects.

Adverse Reaction vs. Side Effects

The terms adverse reaction and side effects are often used interchangeably as well, but tend to have some differences. Side effects are often expected, and often refer to symptoms that are less harmful than adverse reactions.

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.

Adverse Reactions vs. Adverse Events

In contrast to adverse reactions, an adverse event is a negative medical event that occurs while a person is taking a medication, that may or may not be related to the medication.


According to a learning module provided via collaboration by the FDA and the Center for Research and Education on Therapeutics (CERT), there are over two million serious adverse drugs reactions in the United States each year, with 100,000 deaths.


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 reactions have been divided into several basic types:

  • Augmented (dose-related): An example would be bleeding on blood thinners
  • Bizarre (non-dose-related): An example would be a severe allergy (anaphylaxis) to an antibiotic
  • Chemical (dose-related and time-related): An example would be a reaction that could be predicted based on the chemical nature of the drug, such as liver damage to a drug (or breakdown product of a drug) that is toxic to the liver
  • Delayed (time-related): An example would be the development of a secondary cancer related to the carcinogenic effects of chemotherapy or radiation for a first tumor
  • Exit (withdrawal or end of use): An example would be seizures that occur after stopping an anti-seizure drug
  • Failure (failure of therapy) or F (familial)
  • G (genotoxicity)
  • H (hypersensitivity)

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.

Signs, Symptoms and, Severity

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


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 healthcare provider, 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 drug reactions are broken down into four categories based on severity:

  • Mild: No treatment is needed (an example would be sleepiness after taking an allergy medication)
  • Moderate: A change in treatment is needed, either discontinuation of the drug, or use of another drug to counteract the effect (an example might be blood clots on birth control pills, and hospitalization may be needed)
  • Severe: The drug must be stopped and treatment may be needed for a potentially life threatening reaction (an example might be a heart arrhythmia on some medications)
  • Lethal: The adverse reaction results in death (an example would be liver failure due to an acetominophen overdose)


The treatment of an adverse drug reaction will depend on both the symptoms present and the severity of the reaction. If the reaction is mild or moderate, the drug may not need to be discontinued, but the benefits and risks will need to be carefully weighed.

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 healthcare provider 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 practitioner

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

Questions Your Healthcare Provider Will Ask

There are a number of variables that can help your practitioner determine if you are having an adverse reaction to a drug or not. This can be more challenging if you have a medical condition that can cause symptoms similar to those of a medication. She will want to know:

  • The name of the drug, and whether you have switched between different name brands or to a generic
  • The dose you are taking, and if there are any recent changes (increase or decrease in dose)
  • When you started the medication and the time you took the last dose
  • Any history of similar reactions in the past
  • Any other medications you are taking, including any recent changes (drugs you have started or stopped)
  • Any over-the-counter or dietary supplements you have used

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 (erlotinib) 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 healthcare provider will need to weigh the benefit of the treatment against the adverse reaction from the drug.

A Word From Verywell

Adverse drug reactions are common, and are a major cause of hospitalization and death in the United States. With any medication you take it's important to discuss the risks and benefits with your healthcare provider and to promptly report any new symptoms you experience; whether or not you believe they may be related to the drug.

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  1. U.S. Food and Drug Administration. Preventable Adverse Drug Reactions: A Focus on Drug Interactions. Updated 03/06/18.

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