Anticoagulation (Warfarin Service)

About Warfarin

You are likely to have been prescribed Warfarin or Sinthrome to help reduce the risk of harmful blood clots forming in your blood vessels. It is often referred to as a “blood thinner” but it actually works by slowing down your body’s clotting process. The aim is to give just the right amount of Warfarin, so the blood takes longer to clot but does not stop clotting altogether. For this reason, regular blood tests are required.

Blood testing

Your Warfarin treatment will be monitored by having regular blood tests called an INR. INR stands for International Normalised Ratio. The INR is a standard test that measures how long the blood takes to clot. If you are not taking Warfarin or Sinthrome your INR would be expected to measure between 0.9-1.2. When you are prescribed Warfarin or Sinthrome the INR is kept at a higher level, therefore slowing down the clotting process. Your INR target range will vary according to your diagnosis and reason for taking Warfarin or Sinthrome. This will all be discussed when you are introduced to our service.

Key Info

Contact Us

Our service is open Monday to Friday 9am -6pm (please note phone lines are open between 9.30am-5.30pm) There is no service at weekends and over bank holidays.

Tel: 01743 261119


Manager: Sr Alison Pym

Warfarin tablets

0.5mg white tablets

1mg Brown tablets

3mg Blue tablets

5mg Pink tablets

Please note there is no set dose for Warfarin and your Warfarin dose will adjust depending on your INR reading. It is important to have both 3mg and 1mg strength tablets of warfarin on repeat prescription as you may have to use both strength tablets to make up your daily dose. It is important to note that we dose in milligrams (mg) and NOT in the number of tablets.

About our service

The Warfarin service is a nurse led service providing dose advice for patients taking Warfarin and Sinthrome. The team is based at Royal Shrewsbury Hospital and provides a service to patients throughout Shropshire, as well as parts of Powys and Telford, to GP patients who are on oral anticoagulation therapy. Please note we currently do not offer advice to patients taking DOACS e.g Apixaban, Rivaroxaban, and you will need to contact your GP for advice on these drugs.

The service is run by a team of experienced Anticoagulation Nurses and Clerical Staff. There is a Consultant Haematologist on call daily that nurses can liaise with should they need advice on patients INR results.

Your clinician should complete a referral to our service which will include the reason why you need Warfarin, the duration of your Warfarin treatment and your target INR range. Once this has been received we can then provide education on Warfarin medication, advice on INR blood test and dose advice to patients. After your first INR blood test patients will automatically receive a phone call on the day of their blood test or the day after, which will include an introduction to our service and an explanation about our dose cards.

Dose advice provided is based on the results of patient’s latest INR blood test and it is important that the advised dose of Warfarin is followed. There is no face to face contact with the nurses and the service works by a dose and postal service. Patients should receive their dose cards within 4 working days after their INR test. Please note all urgent INR results that need immediate dose changes are looked at and a nurse will contact you by telephone on the day or day after your INR test, so it is important to keep the service up to date with contact detail changes.

Frequently Asked Questions

At the start of your Warfarin treatment blood testing for your INR will be frequent. Initially, blood tests may be at least weekly. As soon as we stabilise your INR then blood testing will be less frequent. Blood tests can vary between weekly testing to 12 weekly testing depending on your INR results.

Your next INR blood test date will be indicated on your dose card. It is important that you have the blood test on the date, or as close to the date we advise. It is the patients’/carer’s responsibility to organise the INR blood test.

An INR below your target range can put you at risk of blood clots. Your Warfarin dose will be increased if no cause for a low INR is found.

An INR that is high can put you at a higher risk of bleeding. If you INR is high your Warfarin nurse will contact you by phone to discuss and your Warfarin dose will need to be decreased. It is important that you seek urgent medical attention if you experience symptoms of a raised INR such as abnormal bleeding or unexplained bruising.

After your first INR test you will receive a phone call to be introduced to our service and our dose cards will be explained. Subsequent INR results will not necessarily be phoned through to patients. We will only phone patients who have an abnormal INR result that requires an immediate dose change. Please also note that due to the amount of INR blood tests we receive daily we often work a day behind however all urgent abnormal INR results are dealt with the same day.

Please allow 4 working days for your card to arrive in the post. Please follow the advice on your current dose card until your new card arrives. If you have not received your card within 4 working days please call the warfarin nurses.

It is very important for your safety that you take the exact dose advised for you on your dose letter. Do not alter the dose. If you do not agree with your Warfarin dose we would advise you call the Warfarin nurses so this can be discussed. If you have your dose card sent by email, please allow 36 hours for delivery.

Don’t worry! If you forget to take a Warfarin dose and it is before 12 midnight it is ok to take this dose. If you forget and it is after 12 midnight then completely miss this dose. DO NOT DOUBLE DOSE. Continue on your usual dose the next day.  All missed doses need to be documented on your blood form as this can affect the way we dose you.

Warfarin is known to interact with a large amount of prescription drugs, over the counter drugs and alternative remedies. Some can increase the effect of Warfarin and some can decrease the effect of Warfarin. This is why it is very important to have an INR blood test 5-7 days after starting or stopping any new medication.

Known side effects are rare but can include skin rash, nausea, vomiting, diarrhoea and hair loss. We would advise you see your GP as soon as possible if you experience any side effects.

The main side effect of Warfarin happens when you take too much or too little Warfarin.

Too much can cause increased risk of bleeding. We would advise you seek urgent medical attention if you show symptoms of a raised INR.

Too little puts you at increased risk of clotting. There are no specific warning signs of a low INR.

It is the responsibility of the clinician who is performing your procedure to give advice on whether your Warfarin can continue as normal. You will need to obtain advice from your clinician regarding your Warfarin before and after your procedure. Please note if your Warfarin is stopped for a procedure the Warfarin nurses will NOT be able to advise on restarting and you will need to contact the department who performed the procedure for this advice.

We will try to avoid INR blood testing whist you are away but it is not always possible and will depend on the result of your recent INR tests. If you have been advised to have an INR test whilst away you can phone your INR result to 01743 261119 (Mon-Fri 9.30-17.30) and we will dose you over the telephone. Alternatively, you can email results to but you must provide a contact number within this email.

If you drink alcohol it is best to drink alcohol in moderation and we advise that you try to stick to the recommended intake. We advise you avoid binge drinking. Please remember Warfarin and large amounts of alcohol are a potentially dangerous combination that can put you at risk of a serious bleeding event.

Yes. Please note our database is not linked to the main hospital records, GP surgeries or any other external hospital. We therefore rely on you to provide current and accurate information about yourself to allow us to provide safe dosing advice.