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 kind 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.

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Always check the potential adverse effects on the label of any medications.

The National Cancer Institute (NCI) define 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.

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 encourage 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.

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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.

Drug-herb interactions can also happen, for example, using antidepressant medication with St. John’s Wort can provoke a hyperactive mood in a person with bipolar disorder.

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.

Some common examples mild adverse effects related to drugs include:

Examples of more serious effects include:

Some effects are more likely than others. The PIL that comes with a drug or device will categorize effects according to their probability.

For example, research may have shown that a headache occurs in 1 in every 10 patients who use the drug according to the instructions. Heart palpitations may occur in 1 in every 100 patients.

It is important to check the PIL on getting new drugs, to find out what effects may occur and how likely these are. The PIL will also advise on whether an effect is classed as an emergency or not.

In the U.S., DailyMed provide information about drugs and their effects through the FDA. It offers an online source for patients to check for details of their medications.

Side effects from vaccinations

Side effects may occur as a result of a vaccine. The effects will depend on the type of vaccine.

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Vaccinations can carry side-effects, but it is worth speaking to your doctor to gauge the risk because of their preventive benefits.

Common vaccine side effects include:

  • Fever
  • A general feeling of being unwell, known as malaise
  • Skin reactions or pain at the vaccination site

Rarely, a person may have an allergic reaction to a vaccine.

The Centers for Disease Control and Prevention (CDC) estimate that this occurs in 1 in every 1 million vaccines and that the reaction usually appears within a few minutes to a few hours after the vaccination.

The CDC suggest that if a person feels faint after having a vaccination, they should lie down and rest for 15 minutes.

Are vaccines a good idea?

Some parents worry about long-term adverse effects of some vaccines, even if these have not been fully proven by research.

The fear of side effects causes some parents not to have their children vaccinated.

However, serious and fatal diseases can result from not having a vaccination. Also, the fewer people have the vaccination, for example, against measles, the greater the threat to public health and the chance of an epidemic. In an epidemic, those who have not been vaccinated will be most at risk.

It is important for parents to weigh up the pros and cons of vaccinating or not vaccinating their children.

Some cancer treatments, such as chemotherapy radiotherapy, or a combination, aim to destroy a tumor or reduce its size. The treatment can cause fatigue, nausea, vomiting, hair loss, mouth sores, and a lower blood cell count. These are adverse effects.

Radiation therapy

Radiation therapy kills unwanted cells, but it can also damage healthy cells, so adverse effects are common.

How severe they are and how long they last will depend on which part of the body is targeted, the radiation dose, and how quickly the damaged cells can recover.

Side effects may include:

  • Fatigue, possibly due to anemia
  • Diarrhea, especially if treatment is for the abdomen. Symptoms tend to appear a few days after the start of treatment and will go away a few weeks later
  • Heart disease, if the site of the tumor is near the heart, for example, in breast cancer treatment
  • Nausea can occur at any time during treatment, or shortly afterward
  • Muscle and joint stiffness
  • Swelling in the affected area
  • Sore skin and skin burns
  • Drop in sex drive and infertility or early menopause, especially if treatment is for the pelvic area
  • Loss of appetite and difficulty swallowing, especially if treatment is aimed at the head, neck, or chest
  • Dry mouth, with treatments aimed at the head, neck or mouth
  • Alopecia, or hair fall, which is normally temporary

Side effects from chemotherapy

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Chemotherapy is known for its severe side effects.

Most people link chemotherapy with uncomfortable side effects, but the management of adverse effects has improved considerably in the last 20 years.

Many side effects that were once inevitable can be either prevented or well controlled today.

Possible side effects include, but are not limited to:

  • alopecia, or hair loss, usually temporary
  • cognitive problems, or problems with thinking, such as attention span, memory,
  • comprehension, reasoning, judgment, and multitasking
  • diarrhea or constipation
  • fatigue
  • hearing impairment
  • infertility
  • loss of appetite
  • low blood platelet count and blood clotting problems
  • low red blood cell count, leading to anemia
  • low white blood cell count, increasing the patient’s susceptibility to infections
  • moodiness
  • mucositis, or inflammation of the mucous membrane
  • nausea and vomiting
  • reduced libido, or less interest in sex
  • dry, sore skin
  • brittle and flaky nails

Some people who are experiencing the late stages of cancer may decide not to undergo chemotherapy or radiation therapy, as they feel the unwanted effects may compromise the quality of their remaining life.

However, in the early stages, and even sometimes in the later stages, these treatments can successfully remove cancer or reduce symptoms and discomfort for some time.

Diagnostic procedures may be invasive or non-invasive. Adverse effects can include allergic reactions, bleeding, or perforation of the intestinal wall, for example, during a colonoscopy.

There is a small risk that a cancer biopsy may cause some of the cancer to break off, enabling it to spread beyond the immediate tumor area. This is called “seeding” of the tumor.

Surgery can cause complications, which are similar to adverse effects.

Depending on the surgery, common complications include:

Following the doctor’s advice after surgery, and when taking any kind of medication, can help reduce the risk of adverse effects.

Being aware of possible adverse effects can help a patient decide to go ahead with some treatments.

During treatment, knowing what adverse effects may occur equips a patient to look out for possible problems, and to deal with them appropriately if they occur.