Can I take Warfarin and Amoxicillin together?

Why this happens

Warfarin works by blocking vitamin K, which the body needs to clot blood. Some of this vitamin K is produced by 'friendly' bacteria in your gut. Amoxicillin can kill these bacteria, leading to lower vitamin K levels and making the warfarin effect much stronger. Amoxicillin may also slightly interfere with how the liver processes warfarin.

What you should do

You must inform the healthcare professional who manages your warfarin (such as your GP or anticoagulant clinic) that you have started amoxicillin. They will likely need to perform an extra blood test (INR test) within 3 to 5 days of starting the antibiotic to check if your dose needs adjusting. Seek medical help immediately if you notice blood in your urine or stools, or if you have a nosebleed that won't stop.

Important precautions

Bleeding risk

Warfarin increases your risk of serious bleeding. You must seek immediate medical attention if you have a fall, a head injury, or notice signs of internal bleeding such as dark/tarry stools or coughing up blood.

Drug interaction

Taking amoxicillin while on warfarin can further increase your risk of bleeding. You will need more frequent blood tests (INR) to monitor your clotting levels while taking this antibiotic.

Pregnancy

Warfarin can cause serious harm to an unborn baby. You must use effective contraception while taking it and inform your doctor immediately if you think you are pregnant.

Penicillin allergy

Do not take amoxicillin if you have ever had an allergic reaction to penicillin. A severe reaction (anaphylaxis) can be life-threatening.

Food and drink warnings

Alcohol

It is best to avoid or strictly limit alcohol while taking warfarin. Alcohol can affect how your liver processes the medicine, which may increase your risk of bleeding or cause your INR levels to become unstable. If you do drink, keep it to a minimum and do not binge drink.

It is best to avoid or strictly limit alcohol while taking amoxicillin. While there is no direct chemical interaction, alcohol can dehydrate you and interfere with your sleep, which may slow down your recovery from infection. Additionally, both alcohol and amoxicillin can cause stomach upset or nausea, and combining them may make these side effects worse.

Foods high in vitamin K (e.g. spinach, kale, broccoli, Brussels sprouts, asparagus)

Vitamin K helps your blood to clot, which works against warfarin. You do not need to avoid these foods entirely, but you must keep the amount you eat consistent. A sudden increase or decrease in vitamin K intake can change how well your warfarin works.

Cranberry juice and Goji berries

Cranberry juice and goji berry products can significantly increase the blood-thinning effect of warfarin, leading to a higher risk of bruising or bleeding. It is best to avoid these while taking warfarin.

Grapefruit juice

Grapefruit juice may increase the levels of warfarin in your blood, which can increase your risk of bleeding. It is best to avoid grapefruit juice.

Pomegranate juice

Some evidence suggests pomegranate juice may interact with warfarin and increase the risk of bleeding. It is best to avoid it.

Using other medicines

Taking multiple medicines? Our Medicines Interaction Checker helps you check whether your prescriptions, over-the-counter medicines, and supplements are safe to take together. Simply search for your medicines to see potential interactions and what to do about them.

Disclaimer

This information is for general educational purposes only and should not be relied upon as a substitute for professional medical advice. Always consult your GP, pharmacist, or another qualified healthcare professional before making decisions about your medications. Individual circumstances may vary, and only a healthcare professional who knows your medical history can provide personalised guidance.