Interferon is a protein released by the cells of the body when they are under attack from a virus or other invader. When interferon is released, it sets off a series of reactions in nearby cells to help them defend against the infection.

Interferon is, therefore, a critical part of the immune system.

There are three basic forms of interferon. These are alpha and beta, also known as type 1, while gamma is known as type 2. Each form of interferon has different effects on the body.

As well as occurring naturally in the body, interferon is also used as a treatment for various health disorders, including multiple sclerosis (MS), some types of cancer, and hepatitis C.

The side effects of interferon can be grouped into three categories, according to Cancer Research UK. These are as follows:

Common side effects

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Side effects, such as a sore throat or nausea, may be mistaken for symptoms of another problem.

Common side effects are those that affect more than 10 percent of users. They include:

  • dizziness
  • pain, redness, or swelling at the point of injection
  • loss or thinning of hair
  • reduced appetite and weight loss
  • breathlessness and pale skin
  • being more likely to bruise and bleed
  • being more likely to pick up an infection
  • exhaustion and weakness
  • flu-like symptoms
  • stomachache
  • diarrhea
  • nausea
  • depression and anxiety
  • insomnia
  • sore throat
  • headache
  • cough
  • joint and muscle pain
  • skin rash

Occasional side effects

Occasional side effects affect between 1 and 10 percent of people. They include:

  • drowsiness
  • changes to the liver
  • heart muscle damage
  • allergic reactions
  • loss of fertility, including possible early menopause
  • periods stopping
  • thirst and dry mouth
  • raised blood pressure
  • worsening migraines
  • decreased sex drive
  • swollen lymph glands
  • indigestion
  • constipation
  • flushed skin
  • breast pain
  • vaginal dryness
  • testicle pain
  • a metallic taste to foods
  • sore and reddened eyes
  • blocked nose
  • sleepwalking
  • sweating more
  • urinating more often

Rare side effects

Rare side effects affect less than 1 percent of people. They include:

  • shakes
  • pins and needles

Some side effects are more serious than others. Here is a look at some of the more severe side effects and what should be done about them.

Depression

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Depression is a serious and common side effect of interferon treatment.

One of the common and potentially most serious side effects of treatment with interferon is depression.

A 2009 review on the subject found that the risk factors for developing depression during interferon treatment include:

  • poor sleep quality
  • chronic inflammation in the body
  • certain genetic factors
  • a lack of social support from others

The review found that using antidepressants along with interferon might be helpful in staving off bouts of depression.

Unfortunately, depression caused by interferon use is likely to become a long-term, recurring condition, according to a 2016 study published in Translational Psychiatry.

Infertility and early menopause

Infertility is an occasional side effect of interferon use and can affect either men or women.

Male infertility from interferon might be reversed after some months or years. However, female infertility will usually be a result of the drug triggering early menopause. As such, it will be permanent.

Anyone hoping to become a parent who is due to have interferon treatment should speak to their doctor about freezing their sperm or eggs before the procedure.

Heart damage

Some past research has found evidence that interferon might lead to heart damage.

One 2004 study looked at the case of a 56-year-old man with normal heart function, who began to experience heart problems, following interferon treatment.

Another study, which was done in 2012, looked at the effects of interferon on increasing risk of heart disease in people with lupus. The researchers found that all subjects, regardless of lupus, seemed to have increased signs of potential heart damage from using interferon.

At the same time, a 2004 study looked at the effects of interferon on the heart health of people with chronic hepatitis. It found no significant negative effects. The authors suggested that interferon therapy might be used safely on people who did not have pre-existing heart disease.

The connection between interferon and heart damage is not clearly understood, but it is worth being mindful of when someone is considering treatment.

Anyone who is concerned, especially those with a history of heart disease, should discuss this with their doctor before starting interferon treatment.

High blood pressure

A 2010 review published in Digestive Diseases and Sciences, looked at four cases where people had received long-term interferon treatment. In each case, interferon seemed to cause irreversible high blood pressure in people with hepatitis C.

The authors noted that this particular issue was rare and had not previously been reported. But it was, nonetheless, a possible side effect of interferon to keep in mind.

The study noted that the underlying risk factors and causes of this issue were unclear, but it occurred during a long course of interferon treatment.

Interferon is usually given by injection underneath the skin of the thigh or belly. The drug may also be given through a drip.

In a 2013 study, interferon was given orally to see whether this would be effective in preventing chest infections. The results suggested this method did not work, although it might have reduced the infection's symptoms.

Older research from 2003 found interferon to have no effect when taken by mouth.

Before undergoing interferon treatment, people should have a lengthy and in-depth conversation with their doctor about what to expect, regarding side effects and symptoms.

After starting treatment, the right time to get in touch with a doctor is whenever these side effects present a real problem to well-being or interfere with someone's quality of life.

A doctor who knows about any side effects may be able to help address these with medication, adjust the treatment plan, or offer advice on counseling and other services.