Warfarin for the treatment and prevention of thrombosis

This leaflet is for parents and carers about how to use this medicine in children. Our information may differ from that provided by the manufacturers, because their information usually relates to adults. Read this leaflet carefully. Keep it somewhere safe so that you can read it again.

Name of medicine

Warfarin (also known as warfarin sodium)

Brand name: Marevan®

This leaflet is about the use of warfarin for the treatment and prevention of blood clots (thrombosis). The use of warfarin to prevent blood clots is called thrombosis prophylaxis.

Why is it important for my child to take warfarin?

In some conditions that affect the heart or blood, blood clots can form more easily than normal and may block blood vessels. Warfarin is an anticoagulant that stops blood clots from forming. This is sometimes called ‘thinning the blood’.

What is warfarin available as?

Tablets: 500 micrograms (white), 1 mg (brown), 3 mg (blue), 5 mg (pink).

Liquid medicine: 5 mg in 5 mL; this can be ordered specially from your pharmacist.

When should I give warfarin

Warfarin is usually given once each day. This is usually in the evening at 6 pm.

Give the medicine at the same time each day so that this becomes part of your child’s daily routine.

How much should I give?

Your doctor will work out the amount of warfarin (the dose) that is right for your child. The dose will be shown on the medicine label.

Your doctor will do regular blood tests to make sure that your child’s blood is clotting properly. This is measured using the INR. They may change the dose of warfarin to get the INR right. Your doctor or nurse will record the warfarin dose and INR in a little yellow book, called an anticoagulation book. You should take this book with you to all your appointments.

It is important that you follow your doctor’s instructions about how much to give.

How should I give warfarin?


  • You can crush the tablet and mix it with a small amount of soft food such as yogurt, jam or mashed potato. Make sure your child swallows it straight away, without chewing.
  • Tablets should be swallowed with a glass of water, squash or juice. Your child should not chew the tablet.

Liquid medicine

Measure out the right amount using a medicine spoon or oral syringe. You can get these from your pharmacist. Do not use a kitchen teaspoon as it will not give the right amount.

When should the medicine start working?

The medicine will start working straight away, although you will not see any difference in your child. Your doctor will check your child’s INR and tell you if the dose needs to be changed.

Do not change the dose without talking to your doctor first.

What if my child is sick (vomits)?

If your child is sick less than 30 minutes after having a dose of warfarin, give them the same dose again.

If your child is sick more than 30 minutes after having a dose of warfarin, you do not need to give them another dose. Wait until the next normal dose.

What if I forget to give it?

If you remember before bedtime, give the missed dose. You do not need to wake a sleeping child to give a missed dose. Do not give the missed dose the following morning, just give the next dose in the evening as usual. If you miss more than one dose, contact your doctor.

Tell your doctor at your next visit if your child has vomited a dose and you have repeated it, or if they have missed a dose.

Never give a double dose of warfarin.

What if I give too much?

If you think you may have given your child too much warfarin, contact your hospital doctor or their team or NHS Direct (111 in England and Scotland, 111 or 0845 4647 in parts of Wales) or take your child to hospital.

Take the medicine container or packaging with you, even if it is empty. This will be useful to the doctor. Have the medicine or packaging with you if you telephone for advice.

It can be dangerous to give too much warfarin, as this can cause bleeding that may be difficult to stop.

Are there any possible side effects?

We use medicines to make our children better, but sometimes they have other effects that we don’t want (side effects).

Side effects you must do something about

If your child has nosebleeds, unusual bleeding from any other part of the body, bleeding that is difficult to stop, or unexplained or unusual bruising, the dose of warfarin may be too high. Contact your doctor or telephone NHS Direct straight away.

If your child’s stool (poo) looks black or red, or if their urine is dark orange or red, this may be a sign of bleeding inside the body. Contact your doctor or telephone NHS Direct straight away.

Other side-effects you need to know about

  • Your child may have stomach cramps, or feel sick or be sick (vomit) when they first start to take warfarin. Giving the medicine with some food may help. If it is still a problem after a week, or you are worried, contact your doctor, but continue to give warfarin.

  • Your child may get a rash. If you are concerned, contact your doctor.

  • Your child’s hair may become thinner and some may fall out. It should grow back when the medicine is stopped. Discuss this with your doctor at your next visit.

Your child may sometimes get side-effects that are not listed above. Contact your doctor if you notice anything unusual or are concerned. You can report suspected side-effects to

Can other medicines be given at the same time as warfarin?

  • You can give your child medicines that contain paracetamol, unless your doctor has told you not to.

Do not give you child medicines that contain ibuprofen or aspirin unless advised by a doctor.

warfarin should not be taken with some medicines that you get on prescription. It is important to tell your doctor and pharmacist about any other medicines your child is taking before starting warfarin.

  • Check with your doctor or pharmacist before giving any other medicines to your child. This includes herbal and complementary medicines.

Is there anything else I need to know about this medicine?

If your child starts to bleed and it will not stop, call an ambulance or take your child to hospital straight away. Make sure that your child’s school and anyone else who may look after them knows to do this.

When you get a new prescription of liquid medicine, check what strength medicine you have and how much to give your child, as it may be different from the previous batch.

  • Your child will be given a card which says that they are taking warfarin. Your child should have this card with them at all times.
  • Make sure that every doctor, nurse or dentist your child sees knows they are taking warfarin.
  • Keep all your clinic appointments, as your doctor or nurse needs to check your child’s INR to make sure they are on the right dose of warfarin. It is important to make sure their blood does not clot too easily, but will clot when it needs to, e.g. if they have a cut or graze. Take your child’s anticoagulation book with you to all appointments.
  • Warfarin may harm an unborn baby. If your daughter is sexually active, it is very important that she uses adequate contraception to prevent unplanned pregnancy. The oral contraceptive pill can be used safely in woman or girls taking warfarin. If your daughter is worried that she may be pregnant, it is important that she sees your family doctor as early as possible. She should keep taking her medicine until she sees her doctor.

General advice about medicines

  • Try to give medicines at about the same times each day, to help you remember.
  • If you are not sure a medicine is working, contact your doctor but continue to give the medicine as usual in the meantime. Do not give extra doses, as you may do harm.
  • Only give this medicine to your child. Never give it to anyone else, even if their condition appears to be the same, as this could do harm.
  • Make sure that you always have enough medicine. Order a new prescription at least 2 weeks before you will run out.
  • Make sure that the medicines you have at home have not reached the ‘best before’ or ‘use by’ date on the packaging. Give old medicines to your pharmacist to dispose of.

If you think someone else may have taken the medicine by accident, contact your doctor straight away.

Where should I keep this medicine?

  • Keep the medicine in a cupboard, away from heat and direct sunlight.
  • It does not need to be kept in the fridge.
  • Make sure that children cannot see or reach the medicine.
  • Keep the medicine in the container it came in.

Who to contact for more information?

Your child’s doctor, pharmacist or nurse will be able to give you more information about warfarin and about other medicines used to treat or prevent blood clots.

England: NHS 111

Tel 111

Scotland: NHS 24

Northern Ireland: NI Direct

Wales: NHS Direct

Tel 111 (free) or 0845 46 47 (2p per minute)

Copyright disclaimer

Version [1]. © NPPG, RCPCH and WellChild, all rights reserved. Review by July 2015.

The primary source for the information in this leaflet is the British National Formulary for Children. For details on any other sources used for this leaflet, please contact us through our website,

We take great care to make sure that the information in this leaflet is correct and up-to-date. However, medicines can be used in different ways for different patients. It is important that you ask the advice of your doctor or pharmacist if you are not sure about something. This leaflet is about the use of these medicines in the UK, and may not apply to other countries. The Royal College of Paediatrics and Child Health (RCPCH), the Neonatal and Paediatric Pharmacists Group (NPPG), WellChild and the contributors and editors cannot be held responsible for the accuracy of information, omissions of information, or any actions that may be taken as a consequence of reading this leaflet.