Side effects happen when a treatment causes a problem because it does more than treat the target issue. The impact can range from minor to severe and life-threatening.
A side effect can, theoretically, be positive. For example, laser treatment for cataracts sometimes improves a person’s eyesight.

An adverse effect, or adverse event, means an unwanted side effect.

The treatment may be a medication, surgical procedure, or other kinds of intervention, including complementary and alternative therapies.

Adverse effects can vary for each patient, depending on their general health, the state of their disease, age, weight, and gender. They can be mild, moderate, or severe.

What are the side effects?

Always check the potential adverse effects on the label of any medications.
The National Cancer Institute (NCI) defines an adverse effect as “an unexpected medical problem that happens during treatment with a drug or other therapy.”

Unwanted effects can result from a physician’s advice and from medications or treatments, including complementary and alternative therapies. They can lead to complications.

Reports from clinical trials describe adverse events (AEs) and serious adverse events (SAEs). SAEs include death, birth defects, complications that require hospitalization, or permanent damage.

Side effects from medications
Any medication can have an adverse effect, whether a prescription drug, an over-the-counter (OTC) drug, an alternative, herbal or complementary therapy, or a vitamin supplement.

For a medication to get approval by the United States Food and Drug Administration (FDA), or a similar body in another country, the drug manufacturer has to list all its known adverse effects.

Adverse effects must be reported, investigated in human clinical trials, and included in the patient information leaflet (PIL). The PIL accompanies drugs and medical devices when they are sold to the public.

The FDA encourages people to report adverse effects to medications.

Adverse effects can result from non-compliance, or non-adherence, which is when the patient does not follow the doctor’s instructions.

Examples include:

not taking a medication that a doctor has prescribed
discontinuing an exercise to strengthen a limb because the activity resulted in pain
Adverse effects of medications are most likely to happen when a person first uses the drug, when they stop using it, or when the dosage changes.

What causes an adverse effect?
There are different reasons for side effects linked to drugs.

These include:

dosage, which may need adjusting
an individual reaction to an ingredient in the drug
a drug killing one type of unwanted cell but also destroying healthy cells
interactions between drugs
A drug interaction happens when another substance affects the activity of a drug. This could be, for example, another drug, a food, a vitamin or supplement, or an essential oil.

Not all side effects are bad, but adverse effects can occur with some medication.
The other substance may increase or reduce the effect of a drug. Sometimes it may cause a completely different action to occur.

Drug-drug interactions happen when two drugs interact. For example, aspirin and warfarin are both blood thinners. Together, they increase the risk of bleeding and bruising.

Drug-food interactions occur when a food alters what the drug should be doing. For example, statins reduce cholesterol levels, but eating high-fat foods will increase them.

OTC preparations, such as aspirin, can trigger drug interactions. It is important to tell a health care professional which drugs you are already taking, including supplements and OTC drugs, at the time of getting a new medication.

In countries where a wide range of drugs can be bought without prescriptions, the risk of drug-drug interactions is greater.

It is important to note that adverse effects from drugs can vary widely, from mild nausea to death. Different drugs have different effects.

Types of effect
Some common examples of mild adverse effects related to drugs include:

Skin rash or dermatitis
Dry mouth
Examples of more serious effects include:

Suicidal thoughts
Abnormal heart rhythms
Internal bleeding
Some effects are more likely than others. The PIL that comes with a drug or device will categorize effects according to their probability.