How often do side effects occur, ranging from common to rare?

The chance of having a drug side effect generally falls in these ranges:

• Very common: affecting more than 1 in 10 people (>10%)
• Common: affecting between 1 in 10 people to 1 in 100 people (1% to 10%)
• Uncommon: affecting between 1 in 100 to 1 in 1,000 people (1% and 0.1%)
• Rare: affecting between 1 in 1,000 to 1 in 10,000 people (0.1% to 0.01%)
• Very rare: affecting less than 1 in 10,000 people (< 0.01%)

Every drug can have side effects and these may range from mild to severe. Most people only experience a small number of drug side effects listed for a drug. The frequency of a drug side effect can help healthcare providers communicate the risk vs. benefit of a drug therapy to patients. This information can help you and your doctor make treatment decisions.

This information on side effect frequencies is a general guideline. Definitions may differ between agencies and countries. For example, the European Medicines Agency (EMA) and the US Food and Drug Administration (FDA) might use different definitions.

Learn more: Risks vs. Benefits of Medications

Examples of side effect frequency

What does this mean in real-world terms? The following examples can help to explain how often side effects occur more accurately:

• If 1 out of 10 people have a headache as a side effect (10%) to a drug, this means that 9 out of 10 people (90%) will not experience a headache due to the medicine. But because it still affects 10% of people, it is considered a common side effect
• If 1 out 500 people (0.2%) have hair loss (alopecia) as a side effect of a medicine, that means that 499 people out of 500 (99.8%) will not be affected by hair loss due to treatment. It is considered an uncommon side effect.
• If 1 out of 10,000 people (0.01%) have an abnormal heart rhythm (arrhythmia) as a side effect of drug treatment, that means 9,999 people (99.99%) will not be affected by this side effect. It is considered a rare side effect.

Related: Side effect information for your medication

How are the frequency of side effects determined?

How often side effects occur due to a drug treatment are typically determined during clinical research studies before a drug is reviewed by the FDA. This information is included in the official prescribing information and can also be found in the patient information leaflet.

The actual frequency of side effects may vary for you based on your age, general health, and other medications used.

In US prescription package labeling, you will often find that the frequency of common adverse events are defined as those occurring in more than 1/100 subjects, infrequent adverse events are those occurring in 1/100 to 1/1,000 patients and rare events are those occurring in less than 1/1,000 patients.

The FDA also gathers side effect information after a drug is approved and marketed. These are called “post-marketing” adverse reactions. Because these reactions are often reported voluntarily from an unknown population, it can be hard to estimate how often they occur or if the event was actually due to the drug treatment itself.

Is a side effect the same as an adverse event?

An adverse event is any undesirable experience associated with the use of a medical product in a patient. This could include medications, devices like pacemakers, or even problems due to surgery.

An adverse drug event is defined by the FDA as an undesirable experience associated with the use of a medicine in a patient. An adverse drug event can be a harmful and negative outcome.

For example, you might have a side effect like an upset stomach after taking an NSAID like ibuprofen, or a rare, serious adverse event like a photosensitivity skin reaction that can occur with the antidepressant Prozac (fluoxetine).

Side effects due to drugs are usually predictable and negative, but in some cases can be beneficial. For example, the side effect of sedation with diphenhydramine (Benadryl) when needed for sleep or weight loss due to type 2 diabetes treatments like semaglutide (Ozempic) can be beneficial effects of the pharmacology of the medicine.

What is a serious adverse event?

Serious adverse events are those that are life-threatening, result in hospitalization or death, can cause disability or result in birth defects. Examples of serious adverse drug events can include severe allergy or anaphylaxis, abnormal heart rhythms, or liver failure.

These events should be reported to the FDA if the event led to death, was life-threatening, resulted in hospitalization, led to disability or permanent damage, caused a birth defect, if a medical or surgical intervention was needed to prevent permanent damage, or if other events like drug abuse or drug dependence occurred.

Your healthcare provider can also submit Medwatch reports to the FDA or offer you assistance. Speak with your doctor, nurse, pharmacist or other provider if you any have questions or concerns about any drug side effect.

How do I report a side effect or negative adverse event?

The US Food and Drug Admininstration (FDA) provides a tool called MedWatch for healthcare providers, consumers and patients to voluntarily report observed or suspected adverse events for human medical products. Voluntary reporting can help the FDA identify unknown risks and side effects for approved medical products.

You can file a Medwatch report with the FDA online, by mail or by calling them at 1-888-INFO-FDA (1-888-463-6332).