Warfarin is an anticoagulant. You might have heard it called a blood thinner.

Warfarin does not actually thin the blood. It lowers the chance of blood clots forming by increasing the time it takes for blood to clot.

Warfarin acts in the liver where it interferes with the process the liver uses to make clotting substances that help blood to clot. The liver usually uses vitamin K to make these substances but warfarin stops the liver using vitamin K properly. By making it harder for the liver to make clotting substances warfarin also makes it harder for the blood to clot. This is useful in preventing unwanted blood clots forming.

The length of time you will need to take warfarin for will vary depending on why you are taking warfarin. It is best to discuss with your doctor why you are taking warfarin and how long you will need to keep taking it.

While everybody is different, the following may be useful as a guide to how long you may expect to be on warfarin:

• Atrial fibrillation: may require life long anticoagulation unless normal heart rhythm is restored.
• Deep vein thrombosis: may require 3 to 6 months of anticoagulation, depending on the patient. If there have been other DVTs or PEs in the past you may need to be anticoagulated for longer.
• Pulmonary embolism: may require 3 to 6 months of anticoagulation, depending on the patient. If there have been DVTs or PEs in the past you may need to be anticoagulated for longer.
• Artificial heart valves: this will depend on the type of heart valve that you have. Generally, a mechanical heart valve will require life long anticoagulation while a biological heart valve will only require short term anticoagulation of up to 3 months.

If you miss a dose of warfarin it is important to know what to do.

If you only just missed your warfarin: If it is only a few hours since you missed your warfarin dose then you should take your usual dose at once. If you take your warfarin with your evening meal and miss a dose, the missed dose can be taken any time before bedtime.

If you missed your warfarin yesterday: If it is more than a few hours since the missed dose (for example, the next day), then don't take the missed dose. Instead take the next dose at the usual time. Do not change the dose. Doubling up the dose is dangerous.

If you missed more than one dose: If you missed your warfarin for more than one day, talk to your doctor or healthcare professional for more advice.

INR stands for International Normalised Ratio. The INR blood test is a measure of how long your blood takes to clot. In a person not taking an anticoagulant the INR is about 1. The higher the INR, the longer it takes the blood to clot. To make sure warfarin therapy is safe and effective a check must be kept on the time it takes your blood to clot. The INR is the most effective way to keep a check on how warfarin is working in your body.

Initially, you will need a blood test daily or every second day until the right dose of warfarin is found. It takes about a week to find the right dose for most people. Once this happens a blood test is only needed every few weeks. The frequency of tests is different for everybody.

After each blood test your dose of warfarin may be changed to keep the INR within the target range for your condition.

Because warfarin is used to make it harder for the blood to clot, the most common side effect associated with warfarin is bleeding events. The risk of bleeding with warfarin is greatly reduced by keeping the INR within the target range for your condition. Things to keep an eye out for include:

Signs of slight bleeding that you may notice from time to time:
• Gum bleeding while brushing teeth.
• Occasional nose bleeds.
• Easy bruising.
• Bleeding after a minor cut that stops within a few minutes.
• Menstrual bleeding that is a little heavier than normal.

Signs and symptoms of more serious bleeding to watch for include:
• Severe bruising that gets worse.
• Any bleeding that take a long time to stop.
• Unexplained bleeding or bruising.
• Menstrual bleeding that is lots heavier than usual.
• Red or dark urine.
• Red or black bowel motions.
• Coughing up blood, or anything red.
• Bloody or dark stained vomit.
• Severe headache or dizziness.
• Weakness or lethargy.
• Unusual pain or swelling.

There are a couple of rare side effects of warfarin that occur is less than one person for every 100 that take warfarin. These are skin lesions (also called skin necrosis), purple toe syndrome, hair loss, and rash.

If you notice any of the above, or anything else that you think may be related to your medicines, talk to your healthcare professional.

In case of an emergency, call an ambulance on 000

If you get scratched or get a small cut, clean the cut with antiseptic and cover it with an adhesive bandage or pad. It's a good idea to apply pressure to try and help the bleeding stop. Most small cuts should stop bleeding in about 10-20 minutes. If the bleeding hasn't stopped, or slowed, in this time, seek medical advice.

If you sustain a bigger cut, cover with a clean pad and apply pressure to help slow the bleeding. Remember that taking warfarin means you'll bleed more and for longer than you're expecting. Larger cuts require medical assistance to stop the bleeding.  Go to your doctor or a nearby hospital or medical centre.

If you fall, or take a hard knock, you may be bleeding on the inside and not see any blood. It is important to go to the doctor and get checked out. If you fall and hit your head this is especially important in case you are bleeding under your skull.