What side effects can warfarin cause?
Warfarin can cause side effects mainly related to bleeding risk. Because it reduces blood clotting, the most important adverse effects are bleeding-related.
Commonly reported side effects include easy bruising and bleeding from gums or the nose [1]. Some people also notice blood in urine or stool, heavier-than-usual menstrual bleeding, or bleeding that doesn’t stop quickly after a cut [1]. Severe bleeding can occur and needs urgent care.
What are the serious (seek care immediately) warfarin side effects?
Urgent symptoms can include signs of major or internal bleeding, such as:
- Vomiting blood or material that looks like coffee grounds
- Black/tarry stools or red/bloody stools
- Severe headache, dizziness, weakness, or fainting
- Sudden back pain, severe abdominal pain, or swelling in an arm/leg
- Shortness of breath or chest pain
- Uncontrolled bleeding
These can signal dangerous bleeding and require immediate medical attention.
Why does warfarin increase bleeding risk?
Warfarin works by interfering with vitamin K–dependent clotting factors, so the body takes longer to form stable clots. Side effects often become more likely when the International Normalized Ratio (INR) is too high, meaning blood is “too thin.” INR changes can happen due to diet changes (especially vitamin K intake), missed doses, drug interactions, or illness.
What other side effects besides bleeding are possible?
In addition to bleeding, some people report:
- Hair loss (usually temporary)
- Skin reactions or rash
- Stomach upset or nausea
These are less emphasized than bleeding, but they can still be important, especially if they worsen.
How do drug interactions affect warfarin side effects?
Many medications can raise or lower INR, which can change both bleeding risk and clotting risk. Drugs that can increase warfarin’s effect may raise bleeding risk, while drugs that reduce its effect can increase the risk of clot formation (the opposite problem).
If you’re starting, stopping, or changing dose of any medication, it can matter for warfarin safety.
What should patients do if they think they’re having a warfarin side effect?
If symptoms suggest uncontrolled bleeding or major internal bleeding, seek emergency care right away.
For milder issues (like small bruises or minor bleeding), contact the prescribing clinician promptly so INR can be checked and the dose can be adjusted if needed.
Are there people who are more likely to have warfarin side effects?
Bleeding risk tends to be higher in people with factors that raise INR or make bleeding more dangerous, such as advanced age, a history of bleeding, certain medical conditions, and interacting drugs. INR monitoring is especially important for higher-risk patients.
Sources
[1] https://www.drugs.com/sfx/warfarin-side-effects.html