Side effects
Vitamin K antagonists
Almost all medicines can have side effects, but not everyone gets them. Read the leaflet that comes with your packet of medicine for a full list of possible side effects.
Bleeding
The main side effect we are concerned about is bleeding. This is why your INR needs to be closely monitored. A high INR indicates a higher risk of bleeding.
It is normal to bruise more easily if you’re taking VKAs.
Contact your GP or call NHS 111 if you have:
- a minor injury and the bleeding does not stop. Put firm pressure on the site for at least 5 to 10 minutes, using a clean, dry dressing. If the bleeding continues seek medical attention.
- heavy or increased bleeding during your menstrual period
- vaginal bleeding (particularly if you have stopped having periods)
- blood in your vomit (sick)
- blood in your sputum (spit)
- blood (red or black) in your poo (stool)
- blood in your pee (making it pink, red or dark brown)
- severe bleeding from your mouth that will not stop
- a lot of bruises, or a very large or painful bruise
Heavy or continued bleeding is rare, but needs urgent medical attention.
Call 999 or go to A&E if you have:
- heavy or long-lasting bleeding (more than 10 minutes), particularly if you feel unwell, very tired, or faint
- bleeding that sprays, pours or is enough to make a puddle
This includes:
- prolonged nose bleeds (that last more than 10 minutes)
- vomiting blood
- coughing up blood
- unusual headaches or feeling drowsy for no clear reason
- heavy or increased bleeding during your menstrual period
- vaginal bleeding, particularly if you have stopped having periods
Bleeding on the brain needs urgent medical help.
Call 999 if you:
- have unusual headaches or a sudden, severe headache
- have a seizure or fit
- have slurred speech
- have arm or leg weakness or numbness
- start feeling drowsy for no clear reason, or are difficult to wake up
Monitoring your INR
A high INR (usually greater than 7) might need to be reversed using vitamin K. This may also be used to reverse the VKA if you are bleeding. When one medicine is used to reverse an effect, it is called an antidote.
You may be given a tablet of vitamin K, or you might need to take a small amount of liquid from a small bottle, and swallow it. Sometimes the anticoagulation clinic may give you an injection of vitamin K.
If your INR is low, we might ask you to increase the amount of VKA you take. It is very important that you tell your clinic if you have missed a dose.
For people with a very high risk of blood clots, we might suggest an anticoagulant injection. This is low-molecular weight heparin. You would only use this for a few days, to reduce the risk of a blood clot while you wait for your INR to rise.
Injury or major trauma
Make sure that you get immediate medical help if you:
- have a head injury or heavy blow to the head
- cannot stop the bleeding
- are involved in major trauma (such as a road traffic accident)
Call 999 or go to your nearest A&E if:
- you have very heavy bleeding or you feel unwell and think it is related to bleeding
If your bleeding is less severe, you can call 111, your anticoagulation clinic, or your GP surgery for advice.
Contact sports
Being active is a vital part of maintaining a healthy lifestyle. However, you should avoid contact sports such as rugby, boxing and hockey, if you are taking anticoagulants.
We also recommend that you avoid sports that have a high risk of injury, such as skiing. Please talk to your anticoagulation clinic if you want to do these types of sports. It is also important to tell your insurance company as this is not covered in all policies.
Effects on your menstrual cycle
If you are menstruating, anticoagulation medicine is likely to affect your period. You should expect that you will bleed more during your period, and your period may last for longer. This can make you feel more tired and lacking in energy (anaemic).
If you notice this, please tell your clinic as there are ways they can help.
Thrombosis UK has created an animation to help explain how VKAs might affect your periods, and what you should do.
Resource number: 5505/VER1
Last review: January 2025
Next review due: January 2028
