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.

Theoretically, a side effect can 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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The National Cancer Institute (NCI) defines an adverse effect as “An undesired effect of a drug or other type of treatment.” They can range from mild to severe. In some cases, they can be life threatening.

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

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

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 using MedWatch.

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 reactions to medications can happen when a person:

  • starts a new medication
  • stops taking their medication
  • increases or decreases their current dose

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

Interactions

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.

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.

Drug-food interactions occur when a food alters what the drug should be doing. For example, those taking statins may need to avoid consuming grapefruit. Grapefruit can increase the levels of statins in the blood.

Drug-herb interactions can also happen, for example, taking St John’s Wort can lower the effectiveness of birth control pills.

OTC preparations, such as aspirin, can trigger drug interactions. It is important to tell a healthcare professional which drugs a person is 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 of 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.

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 provides 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.

Common vaccine side effects include:

  • fever
  • tiredness
  • body aches
  • skin reactions or pain at the vaccination site

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

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 who 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 speak with a doctor to help weigh up the pros and cons of vaccinating or not vaccinating their children. The CDC and FDA make sure that vaccines are safe.

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 loss, which is normally temporary

Side effects from chemotherapy

Chemotherapy kills cancer cells and travels throughout the body. However, they can also affect healthy cells, which can cause side effects.

The American Cancer Society notes that the side effects are not always as severe as a person might think.

Possible side effects include, but are not limited to:

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 noninvasive. Adverse effects can include allergic reactions, bleeding, or perforation of the intestinal wall, for example, during a colonoscopy.

There is an extremely 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:

  • cardiovascular risks, such as deep vein thrombosis (DVT) and pulmonary embolism
  • changes in local blood flow
  • constipation
  • erectile dysfunction, for example, after the removal of the prostate gland
  • hemorrhage, or bleeding
  • infection
  • inflammation
  • loss of function
  • nerve damage
  • scarring

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 manage them appropriately if they occur.