Warfarin

What is warfarin?

Warfarin is an anticoagulant medicine available on prescription, commonly referred to as a blood thinner. It reduces the blood’s ability to clot, helping to prevent harmful clots from forming or growing larger. It does not dissolve existing clots but can stop them from getting worse[1]. 

What is warfarin used for?

Warfarin is prescribed to prevent or treat serious conditions where blood clots can cause harm. These include[1]:

• Deep vein thrombosis (DVT) – blood clots in the deep veins, usually in the legs 

• Pulmonary embolism (PE) – clots in the lungs 

• Atrial fibrillation (AF) – an irregular heartbeat that increases the risk of stroke 

• Heart valve replacement – to prevent clots forming on artificial valves 

• Stroke or transient ischaemic attack (TIA) – to reduce the risk of recurrence 

• Blood clotting disorders such as Thrombophilia – a condition that increases the risk of abnormal blood clotting

How does warfarin work?

Warfarin works by interfering with vitamin K, which the body needs to produce clotting factors in the liver. By limiting the action of vitamin K, warfarin reduces the frequency and extent to which the blood forms dangerous clots[1]. 

Who can take warfarin?

Warfarin can be taken by adults and sometimes by children under specialist care. However, it may not be suitable for people who: 

• Are pregnant, particularly in the first trimester, or trying to get pregnant 

• Have liver or kidney problems 

• Have endocarditis (an infection of the lining of your heart) 

• Have uncontrolled high blood pressure 

• Are at high risk of falls or bleeding 

• Have certain medical conditions that causes bleeding such as stomach ulcers or makes you bruise easily  

• Have had an allergic reaction to warfarin or any of the other ingredients listed in your medicine 

Talk to your doctor or anticoagulant clinic if you have a planned surgery or dental procedure. You should also inform your doctor or anticoagulant clinic if you lose weight, have an acute illness, kidney problems or increase vitamin K intake as dose may need to be altered. 

Please tell your doctor or pharmacist about any conditions you may have or medication you take before you start treatment, and always follow the instructions in the Patient Information Leaflet[2].

What is the typical dosage of warfarin?

Warfarin is taken once daily, at the same time each day. The dose is tailored to each person and adjusted after having regular blood tests that measure the INR (International Normalised Ratio). The ideal INR range depends on the condition being treated, but it is typically between 2.0 and 3.0. These blood tests will take place at your GP surgery or your local hospital’s anticoagulant clinic[1]. 

The usual warfarin dose for adults is initially 5-10mg a day for 1 or 2 days depending on your condition and age, then between 3mg and 9mg a day depending on your INR. The usual child’s dose of warfarin is based on their weight[1]. 

There are 4 different strengths of warfarin tablets. They come in different colours, both tablets and boxes, to make it easier for you to know what to take. The strengths and colours are[1]: 

• 0.5mg – white tablet 

• 1mg – brown tablet 

• 3mg – blue tablet 

• 5mg – pink tablet 

Your dose may be made up of a combination of different coloured tablets. Warfarin also comes as a liquid, where 1ml is equal to a 1mg (brown) tablet[1]. 

How do you take warfarin?

To take warfarin safely[1]:

• Take it at the same time every day 

• It can be taken with or without food 

• Avoid sudden changes in your diet, especially foods high in vitamin K such as leafy green vegetables 

• Always inform your doctor or pharmacist before starting any new medications or supplements

Can you stop taking warfarin?

Do not stop taking warfarin unless advised by your doctor or pharmacist. Stopping suddenly can increase the risk of blood clots. If stopping is necessary, it will usually be done gradually under medical supervision[1]. 

What do you do if you take too much warfarin?

Taking too much warfarin increases the risk of bleeding. Signs may include[1]: 

• Nosebleeds, bleeding gums, or heavy periods 

• Blood in urine or stools 

• Severe bruising or prolonged bleeding 

Seek medical advice immediately by calling 111 or go to A&E if symptoms are severe. 

What do you do if you forget to take warfarin?

If you forget to take your dose[1]:

• Take it as soon as you remember, unless it is nearly time for the next dose 

• Do not take two doses at once to make up for a missed one 

What medicines can’t you take with warfarin?

Warfarin interacts with many substances. Always check with your doctor or pharmacist before starting anything new. Be especially cautious with[1]: 

• Aspirin and non-steroidal anti-inflammatory drugs (NSAIDs) like ibuprofen or aspirin 

• Heart medicines like amiodarone, quinidine, or propafenone 

• Tramadol 

• Other anticoagulants or antiplatelets (e.g. apixaban, clopidogrel) 

• Some antidepressants (e.g. citalopram) 

• Cholesterol-lowering medication like bezafibrate, gemfibrozil, clofibrate, or cholestyramine 

• Some antibiotics and antifungals (e.g. miconazole, erythromycin, norfloxacin, amoxicillin) 

• Herbal products such as St John’s Wort and food like cranberry 

You should also monitor your alcohol intake and completely avoid binge drinking as can increase bleeding risk. 

Please note this not a complete list, always inform your doctor or pharmacist about all medications and supplements you take before you start a new medicine, and follow the instructions in the Patient Information Leaflet[2]. 

What are the side effects of warfarin?

The most common side effects are related to bleeding and bruising, but others can occur as well. 

The most common side effects include[1]: 

• Hair thinning 

• Skin rashes 

The most serious side effects include[1]: 

• Blood in urine or stools 

• Severe headache or dizziness 

• Coughing up blood 

• Severe back pain 

• Unexplained bruising 

• Vaginal bleeding post-menopause 

• Heavy or prolonged bleeding 

You should contact your GP if you experience any of these symptoms. In rare cases, the medicine may cause a serious allergic reaction (anaphylaxis). You should call 999 or visit A&E immediately if you believe this is happening[1]. 

Symptoms of anaphylaxis include[1]:  

• Difficulty breathing  

• Tightness in throat and chest  

• Swelling of the tongue or throat  

• Feeling faint or dizzy  

• Blue/grey skin or lips  

• Loss of consciousness 

If you experience any side effects, including those not listed, you are encouraged to report them via the Yellow Card Scheme or contact your doctor, pharmacist, or nurse. This information is intended as a summary for general awareness and does not replace professional medical advice. 

The information provided in the side effects section is not a comprehensive list of all possible side effects associated with this medication. For a complete list of side effects, please refer to the Patient Information Leaflet (PIL) provided with your medication or consult your healthcare professional[2].

Frequently asked questions about warfarin

Do I need regular blood tests? 
Yes. INR tests are essential to ensure your dose of warfarin is correct and your blood is clotting within a safe range[1]. 

Can I take painkillers with warfarin? 
Paracetamol is usually safer but avoid NSAIDs like ibuprofen unless advised by your doctor[1]. 

Is it safe to exercise on warfarin? 
Yes but avoid high-risk or contact sports due to the increased risk of bleeding[1]. 

Can I travel while taking warfarin? 
Yes, but carry your medication, INR record, and a warfarin alert card with you[1]. 

Can I have a tattoo or piercing while on warfarin? 
You’re at increased risk of bleeding and infection so it’s not recommended[1]. 

For the official NHS guide to warfarin, visit their website.