Yes, some antibiotics can interact with blood thinners, but the risk depends on which medicines you take. Certain antibiotics can make a blood thinner work more strongly and raise bleeding risk; others can reduce its effect and increase the chance of a clot. Tell the prescriber and pharmacist about every medicine you use. Do not stop your blood thinner or change either dose on your own.
Why Can Antibiotics Interact With Blood Thinners?
Blood thinners are medicines that help prevent harmful blood clots. Some, called anticoagulants, include warfarin, apixaban, rivaroxaban, dabigatran, and edoxaban. Although these medicines serve a similar purpose, they work differently in the body and do not all interact with antibiotics in the same way.
An antibiotic may change how a blood thinner works or how much of the medicine is present in the bloodstream. Depending on the combination, this may increase bleeding risk or make the blood thinner less effective at preventing clots.
The NHS guidance on anticoagulant medicines confirms that certain antibiotics can affect anticoagulant treatment. This does not mean that everyone taking a blood thinner must avoid antibiotics. Many combinations can be used when a healthcare professional has reviewed the medicines and arranged any monitoring that is needed.
Which Antibiotics Can Interact With Blood Thinners?
The interaction depends on the specific antibiotic and the blood thinner involved. The examples below illustrate why checking the exact combination is important.
Antibiotics and warfarin
Warfarin is an anticoagulant whose effect is measured using a blood test called the international normalized ratio (INR). The INR helps show how long blood takes to clot compared with a standard reference. Certain antibiotics can change warfarin's effect, which may cause the INR to move outside the person's intended range.
An INR that becomes too high can be associated with increased bleeding risk, while an INR that is too low may mean less protection against blood clots. The size and direction of an interaction depend on the medicines and the person's circumstances.
MedlinePlus advises people taking warfarin to discuss antibiotics and other new medicines with their healthcare professional. Changes in health, eating patterns, vomiting, or diarrhea can also affect warfarin management, so an infection may require attention beyond simply choosing an antibiotic.
Antibiotics and newer blood thinners
Apixaban, dabigatran, edoxaban, and rivaroxaban are direct oral anticoagulants, often called DOACs. Unlike warfarin, they are not routinely managed using INR measurements. However, some antibiotics can still change their levels or effects.
According to the NHS Specialist Pharmacy Service guidance on DOAC interactions:
- Clarithromycin and erythromycin: These antibiotics may increase the blood levels and anticoagulant effects of DOACs, potentially raising the risk of bleeding.
- Rifampicin: This antibiotic can lower DOAC blood levels and reduce their anticoagulant effect, potentially increasing the risk of a blood clot.
These examples do not represent every possible interaction. The effect can differ between individual DOACs, and the appropriate action depends on the particular combination. Older adults and people with reduced kidney function may need especially careful review for some of these interactions.
What Should You Do Before Starting an Antibiotic?
Before taking a new antibiotic, tell the prescribing healthcare professional that you use a blood thinner. If possible, provide the exact name, strength, and dose of your medicine. Also mention prescription medicines, nonprescription products, vitamins, and herbal supplements you take, because other substances can contribute to interactions.
Use the following steps to help manage your medicines safely:
- Check the combination before the first dose. Ask the prescriber or pharmacist to review your specific antibiotic and blood thinner.
- Ask whether additional monitoring is needed. Some combinations require blood tests or closer observation for bleeding or signs that clot prevention has become less effective.
- Tell your anticoagulation clinic when treatment starts and ends. This is particularly important if you take warfarin, because its effect may change during or after antibiotic treatment.
- Follow the prescribed instructions. Take both medicines as directed, unless your healthcare professional gives you different instructions.
- Keep your medication list updated. Include the antibiotic's name and the planned treatment dates so that other healthcare professionals can review your treatment accurately.
Do not skip doses of your blood thinner, reduce its dose, or stop it to make an antibiotic safer. Doing so may leave you less protected against blood clots. Likewise, do not stop or change a prescribed antibiotic without discussing the decision with the prescriber.
Will You Need an Extra INR Blood Test?
If you take warfarin, your healthcare professional may arrange an additional INR test after you start an antibiotic known to interact with it. The timing depends on the medicines involved and your current INR control.
The NHS Specialist Pharmacy Service guidance on warfarin monitoring recommends checking the INR three to five days after starting a medicine that may interact with warfarin. Your anticoagulation team can determine whether this timing applies to your antibiotic and whether further tests are needed.
If you take apixaban, dabigatran, edoxaban, or rivaroxaban, routine INR testing is not used to guide treatment in the same way as it is for warfarin. Your healthcare professional will instead assess the interaction and decide whether a different antibiotic, a change in treatment, or additional clinical monitoring is appropriate. Do not adjust your dose yourself.
What Signs of Bleeding Should You Watch For?
Because blood thinners reduce the blood's ability to form clots, bleeding can become more likely or take longer to stop. An interaction with an antibiotic may increase this concern in some circumstances.
Examples of possible signs of excessive bleeding include:
- Unexpected or extensive bruising.
- Bleeding from the gums or a nosebleed that lasts longer than usual.
- Red, pink, or brown urine.
- Blood in the stool or black, tar-like stool.
- Vomiting blood or material that looks like coffee grounds, or coughing up blood.
- Unusually heavy menstrual bleeding or other unexpected vaginal bleeding.
- Persistent bleeding from a small cut.
The NHS information on anticoagulant side effects identifies these types of bleeding as reasons for prompt medical attention. If you notice blood in your urine or stool, black stool, or repeated or prolonged bleeding, seek urgent medical assessment rather than waiting for your antibiotic course to finish.
Seek emergency medical care if bleeding will not stop, you vomit blood, you have difficulty breathing or chest pain, or you experience a significant blow to the head while taking a blood thinner. Head injuries warrant urgent assessment even if you initially feel well. Do not wait for your next scheduled blood test if concerning bleeding occurs.
Can You Reduce the Risk of an Interaction?
You cannot prevent every drug interaction yourself, but a few practical habits can help your healthcare team manage the risks:
- Use antibiotics only when prescribed for you, and follow the instructions provided.
- Tell every prescriber and pharmacist that you take an anticoagulant, even when the antibiotic is prescribed for a short course.
- Ask before adding nonprescription medicines, including pain relievers, because some can increase bleeding risk when combined with blood thinners.
- Do not assume an antibiotic is safe simply because you have taken it before. Your current medicines and health circumstances may be different.
- Contact your healthcare professional if you develop vomiting, diarrhea, a new illness, unusual bruising, or bleeding while taking warfarin or another anticoagulant.
The goal is not necessarily to avoid antibiotics. When an infection needs treatment, the prescriber can select an appropriate antibiotic and decide whether the blood thinner requires closer monitoring or another adjustment. The safest plan is one that accounts for both the infection and the medicine used to prevent blood clots.