by | Heart Attacks and Strokes


Anticoagulant medicines reduce the blood’s natural ability to clot. Although it is important for everyone’s blood to clot so that bleeding from cuts and other accidents will eventually stop, in some people it is important to slow the clotting process down with anticoagulants.

Anticoagulants, sometimes referred to as ‘blood thinners’, do not affect any other function of blood, such as carrying oxygen or fighting infection.

Reasons for taking anticoagulants

Anticoagulants are used to prevent and treat blood clots in people who have had a previous blood clot that has caused a health problem or those who are at risk of developing a blood clot.

Common reasons for taking anticoagulant medications:

  • having a history of abnormal blood clotting, for example a clot in the leg (DVT) or on the lung (pulmonary embolism);
  • having an irregular heartbeat, such as atrial fibrillation;
  • having an artificial heart valve;
  • following certain types of surgery (such as knee or hip replacement surgery); and
  • (in some cases only) after a heart attack or stroke.

Precautions when taking anticoagulants

  • Bleeding. You will bleed more easily when taking anticoagulants, even from minor bumps, scrapes or cuts. Take care when shaving or tooth brushing. Bleeding can be serious, so make sure you take your anticoagulants as directed by your doctor and pharmacist, and have regular monitoring with your doctor.
  • Other medicines. Many anticoagulants are affected by medicines that are used to treat other conditions, potentially increasing your risk of bleeding or clotting. Check with your doctor before starting any new medicines, including complementary medicines.
  • Surgery. If you need to have surgery, it’s important that your surgeon and doctor are aware that you are taking anticoagulation medicines. The dose and timing of your anticoagulants will most likely need to be adjusted before surgery.
  • Dental procedures. Let your dentist know that you are taking anticoagulants before any dental procedures that may put you at risk of bleeding.
  • Pregnancy. Some anticoagulant medicines, such as warfarin, are not suitable for use in pregnancy. If you are pregnant or planning a pregnancy, see your doctor.
  • Emergency medical ID. It’s a good idea to wear or carry some medical ID that lists the medicines you are taking, and the name of your doctor. Available options include bracelets (such as MedicAlert) and other jewellery, or cards for your purse or wallet.

Types of anticoagulants

There are several different types of anticoagulants, including:

The type of anticoagulant medicine that your doctor prescribes will depend on several factors, including:

  • the reason for anticoagulation;
  • other medical conditions; and
  • medication side effects.

Anticoagulants can be given through a drip into a vein, by injection under the skin or as a medicine by mouth.

Sometimes, a combination of different anticoagulants is used.


Warfarin is perhaps the best-known anticoagulant medicine. For many years, warfarin (brand names Coumadin, Marevan) was the only oral anticoagulant available. It may be needed for a short time or, in many cases, for life.

People taking warfarin need regular blood tests to check the level of anticoagulation. A measure known as INR (international normalised ratio) has been developed to compare the rate of blood clotting between someone taking anticoagulants and someone who is not.

The INR of a healthy person not taking warfarin is 1.0. The longer it takes to clot, the higher the INR. For most people on anticoagulants, the INR needs to be maintained somewhere between 2.0 and 3.5.

It’s important to keep your INR in your target range:

  • if your INR is below your target range, you are at risk of blood clots developing;
  • if your INR is too high you are at risk of bleeding.

Factors that may affect your INR

  • Vitamin K. Extra Vitamin K can counteract the effect of warfarin. Certain foods, particularly those containing high amounts of Vitamin K such as green leafy vegetables, can therefore affect your INR. It is not that you shouldn’t eat these foods, but your intake should not vary widely. Tell your doctor if you are making major changes to your regular diet. Vitamin K may also be included in multivitamins, so check this with your pharmacist.
  • Alcohol can also affect your INR, so you should limit your alcohol to no more than 2 standard drinks per day and avoid binge drinking.
  • Other medicines. Taking other medicines can also affect the rate of blood clotting in people taking warfarin. Warfarin has more potential for interacting with other medicines than just about any other medication, so always discuss with your doctor or pharmacist before taking any new medicines.

Newer anticoagulants

Several other anticoagulants have become available more recently, including:

  • dabigatran (brand name Pradaxa), taken orally;
  • rivaroxaban (brand name Xarelto), taken orally;
  • apixaban (brand name Eliquis), taken orally; and
  • fondaparinux (brand name Arixtra), which is injected under the skin.

These anticoagulants generally do not require blood test monitoring. They may be unsuitable in people with reduced kidney function.

Unfractionated heparin

Unfractionated heparin is an anticoagulant medicine that is given via a drip, usually in hospital. Its effect on clotting comes on and wears off quickly, and needs to be closely monitored. It is not suitable for long-term anticoagulation.

Unfractionated heparin can cause an uncommon reaction in some people resulting in a reduced number of platelets (cells in the blood involved in clotting) and problems with clotting.

Low molecular weight heparins

Low molecular weight heparins, such as dalteparin (brand name Fragmin) and enoxaparin (brand name Clexane), are more predictable than unfractionated heparin and do need to be monitored as closely.

They are given via injection under the skin, which can be self-administered at home.

Anti-platelet agents

Another type of medicine – antiplatelet agents – is also used to help prevent blood clots from forming. Examples include aspirin and clopidogrel (Iscover, Plavix).