Side effects of TK inhibitors

The side effects of TK inhibitors are often more noticeable when you first start taking them. They usually settle down after the first four weeks of treatment. They will go away once your treatment is stopped.

Your doctor or nurse will explain the possible side effects of your treatment and give you advice on how to manage them. Always let your doctor or nurse know about any side effects you’re having. Sometimes, if side effects are severe, your doctor will suggest that you stop treatment for a few days.

Side effects of TK inhibitors include:

  • feeling sick

  • diarrhoea
  • loss of appetite
  • headaches
  • muscle bone and joint pain
  • fluid retention
  • skin changes
  • tiredness and weakness.

Your doctors and nurse will advise you to use contraception while having treatment with a TK inhibitor. This is because these drugs are quite new and there isn’t a lot of information about what happens if a woman becomes pregnant or a man fathers a child while taking them.

Side effects of TK inhibitors

The side effects of TK inhibitors are usually mild and treatable. They are often more noticeable in the first four weeks of treatment and begin to settle after this.

Side effects generally disappear when treatment is stopped. If you have severe side effects, your doctor may ask you to stop taking the drug for a few days. After a short break, you may be able to start taking it again without having the same problems. Occasionally people need to stop treatment because their side effects are too severe.

Always let your doctor know if you notice any new side effects or your side effects get worse.

Feeling sick (nausea)

This is usually mild. Your doctor may prescribe anti-sickness (anti-emetic) drugs to prevent or reduce this. Let them know if you still feel sick, as they can prescribe another anti-sickness drug that may work better for you.


This can usually be controlled with anti-diarrhoea medicine, but tell your doctor if it’s severe or continues. It’s important to drink plenty of fluids if you have diarrhoea.

Loss of appetite

A dietitian or specialist nurse can give you advice and tips on boosting your appetite, coping with eating difficulties and maintaining your weight.


Let your doctor know if you have headaches, as they can advise you which painkillers to take.

Muscle, bone and joint pain

You may have muscle, joint or bone pain while having your treatment. Your doctor can prescribe painkillers to ease this.

Fluid retention

This can affect different parts of your body. Your ankles may swell, or you may get swelling around the eyes. Fluid retention often settles without treatment. But if it doesn’t, drugs that make you pass more urine (diuretics) can help get rid of some of the fluid. A short course of steroids may also help.

Skin changes

Your skin may become dry and itchy or you may develop a mild skin rash. Making sure you drink enough fluids can help. Your doctor can also prescribe medicine or creams to help with this.

Tiredness and feeling weak

This is common but is usually mild. It’s important to give yourself plenty of time to rest.


Drinking plenty of fluids, eating a high-fibre diet and doing gentle exercise usually helps relieve constipation. Sometimes you may need to take medicines (laxatives) to stimulate your bowel. Your doctor can prescribe these.

Risk of infection

TK inhibitors can reduce the number of healthy white blood cells produced by the bone marrow, making you more prone to infection.

Contact your doctor straight away if:

  • you develop a high temperature, which may be over 37.5˚C (99.5˚F) or over 38˚C (100.4˚F) depending on the hospital’s policy – follow the advice that you have been given by your chemotherapy team
  • you suddenly feel unwell, even with a normal temperature.

Bruising or bleeding

TK inhibitors can reduce the production of platelets, which help the blood to clot. Tell your doctor if you have any unexplained bruising or bleeding, such as nosebleeds, bleeding gums, blood spots or rashes on the skin. They will advise you about this and explain any precautions you should take.

Anaemia (low number of red blood cells)

You may become anaemic. This can make you feel tired and breathless. Tell your doctor or nurse if you have these symptoms. You may need to have a blood transfusion if the number of red blood cells becomes too low.

Your blood count will be monitored regularly while you’re having treatment. If your blood cell numbers fall too much, your doctor may stop your treatment for a few days to let them recover.

Or you may be prescribed injections of substances called growth factors. The growth factor G-CSF can stimulate your bone marrow to produce more healthy white blood cells, and erythropoietin can help with the production of red blood cells.

Contraception and fertility

TK inhibitors are a newer type of drug, so there isn’t a lot of information about women becoming pregnant or men fathering children while taking them. There is a slightly increased risk of damage to the developing baby, so it’s strongly recommended that you use contraception while being treated with a TK inhibitor.

If you think you may want to have children in the future, talk to your doctor before starting treatment.

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