Adverse Reaction to a Medication or Drug

Adverse reactions can range from minor to severe

An adverse reaction is an unexpected negative reaction to a medication or treatment that happens even when used correctly.

You may have adverse drug reactions shortly after taking a medication or maybe decades later. As a leading cause of illness and death in the United States, the importance of adverse reactions cannot be overstated.

Any prescription drug, over-the-counter (OTC) medication, nutritional supplement, and botanical health product can potentially cause adverse reactions.

A Common Problem

Each year, more than two-million people in the United States have serious adverse drugs reactions and about 100,000 people die from them.

woman with skin rash on arm
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What Is an Adverse Reaction?

An adverse drug reaction is defined as an unintended effect of a medication that is harmful or unpleasant. While several other terms are similar, they all have different meanings.

Adverse Reactions vs. Adverse Effects

  • An adverse reaction usually refers to clinical signs and symptoms—what you experience while taking a medication.
  • An adverse effect usually refers to something seen on lab tests or imaging studies.

For example, an adverse effect might be lowering blood sugar. Some people may have an adverse reaction—hypoglycemia—due to that effect. Those with no symptoms or a beneficial effect still have the adverse effect happening, but it doesn't lead to an adverse reaction.

Just as not all adverse effects lead to adverse reactions, not all adverse reactions are related to adverse effects.

Adverse Reaction vs. Side Effects

While sometimes used interchangeably with side effects, the terms have slightly different meanings:

  • An adverse drug reaction is always negative.
  • A side effect is simply a secondary effect of a medication, which may be harmful, neutral, or even beneficial.

For example, some medications that cause fatigue as a side effect may be a problem for some people but could be used as a sleep aid for others.

Adverse Reaction vs. Allergy

A drug allergy is a serious matter. Technically, it's a type of adverse reaction.

In an allergic reaction, your body recognizes the drug as foreign, mistakenly considers it dangerous, and activates the immune system to "fight it off." The immune response is what causes allergy symptoms such as:

  • Rashes or hives
  • Itching
  • Wheezing/breathing problems
  • Inflammation
  • In severe cases, anaphylaxis (a potentially life-threatening reaction)

True allergic reactions to drugs make up about 10% of adverse reactions.

Adverse Reactions vs. Adverse Events

An adverse event is a negative medical event that occurs while you're taking a medication, but that may or may not be related to the medication.

For instance, a medical study may note that one participant had a heart attack while taking a drug. That doesn't necessarily mean that the drug caused it—it may have contributed, but it may also have just been a coincidence.


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 that causes fatigue, in which tiredness worsens sequentially with increasing doses. 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 that 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.
  • They 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 have no symptoms or obvious, even life-threatening symptoms. They can occur immediately after you start treatment or develop over time.

Symptoms of adverse reactions vary by drug, but some common ones include:

  • Gastrointestinal (digestive system) bleeding
  • Heartburn
  • Fatigue/sleepiness
  • Nausea and diarrhea
  • Lightheadedness or dizziness 
  • Constipation
  • Skin rashes

Some adverse reactions are common and can be anticipated by your healthcare provider, whereas others are rare.


The treatment of an adverse drug reaction depends on many factors, including the symptoms and their severity.

Severity is broken down into four categories that determine the basic approach:

  • Mild: No treatment is needed, such as with sleepiness from an allergy medication
  • Moderate: Either the drug can be stopped or another drug added to counter the reaction.
  • Severe: The drug must be stopped and treatment may be needed for a potentially life-threatening reaction.
  • Lethal: The adverse reaction results in death, such as liver failure from an acetaminophen overdose.

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 a healthcare provider.

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

Questions Your Healthcare Provider Will Ask

Several variables can help your healthcare provider determine whether you're having an adverse reaction. This can be more challenging if you have a medical condition that can cause symptoms similar to those of the medication.

Your provider will want to know:

  • The name of the drug, whether it's new, and if you've switched to a different brand or to a generic
  • The dose you're taking and if there are any recent increases or decreases
  • When you started the medication and the time you took the last dose
  • Any history of similar reactions
  • Any other medications you are taking, including any recent changes
  • Any over-the-counter drugs or dietary supplements you've 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. However, the rash seems to indicate the drug is working well. In fact, the more severe the rash, the better the drug's effect.

In these situations, you and your healthcare provider should weigh the benefit of the treatment against the adverse reaction from the drug.

A Word From Verywell

Adverse drug reactions are common and 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.

You should promptly report any new symptoms and potential reactions to your provider, even if you're not sure whether they're related to the drug.

Frequently Asked Questions

  • Who is most at risk of adverse drug reactions?

    People over 65 are most at risk of adverse reactions to medication. They're hospitalized because of them at twice the rate of younger people.

3 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. U.S. Food and Drug Administration. Preventable adverse drug reactions: A focus on drug interactions.

  2. American Collage of Allergy, Asthma, & Immunology: Allergist. Drug allergies.

  3. Lavan AH, Gallagher P. Predicting risk of adverse drug reactions in older adultsTher Adv Drug Saf. 2016;7(1):11-22. doi:10.1177/2042098615615472

Additional Reading

By Lynne Eldridge, MD
 Lynne Eldrige, MD, is a lung cancer physician, patient advocate, and award-winning author of "Avoiding Cancer One Day at a Time."