Based on NIH | Is diarrhea a side effect of warfarin, and when should I seek medical advice if it occurs?

Diarrhea can occur with warfarin, and it’s also a reason to contact your healthcare team because it may affect your INR and bleeding risk. ^{[1] [2]} Official patient instructions advise contacting a clinician if you develop diarrhea while on warfarin, especially if it is severe or lasts more than 24 hours. ^{[3] [4]}

Is diarrhea a known side effect?

• Yes, gastrointestinal side effects including diarrhea are listed in warfarin’s official prescribing information. ^{[1] [2]}
• While not the most common issue compared with bleeding, diarrhea is recognized among warfarin’s GI adverse reactions (alongside nausea, vomiting, abdominal pain, and bloating). ^{[1] [2]}
• In clinical trial summaries comparing anticoagulants, vitamin K antagonists (like warfarin) reported diarrhea in a range of 0–12.4% across studies, indicating it does occur, though rates vary by study. ^[5]

Why diarrhea matters more if you take warfarin

• Diarrhea can change how your body handles vitamin K and warfarin, potentially increasing your INR and bleeding risk. ^{[6] [7]}
• Illnesses such as severe diarrhea, infections, or fever are specifically flagged in warfarin patient guidance as reasons to call your doctor. ^{[8] [9]}
• Antibiotics prescribed for diarrhea or infections can also interact with warfarin and shift your INR, so closer INR monitoring is recommended when starting or stopping these drugs. ^{[10] [11]}
• Some antibiotics (for example, trimethoprim-sulfamethoxazole and fluconazole) are linked with higher risks of gastrointestinal bleeding in people on warfarin, underscoring the need for careful oversight if they’re used. ^{[12] [13]}

When to seek medical advice

• Contact your healthcare provider promptly if you have diarrhea, fever, or symptoms of infection while taking warfarin. ^{[3] [4]}
• Reach out urgently if diarrhea is severe, lasts more than 24 hours, or if you cannot eat or keep fluids down, as this may require INR checks and dose adjustments. ^{[14] [4]}
• Seek immediate help if you notice any signs of bleeding (black or tarry stools, blood in stool, coughing or vomiting blood, unusual bruising, or bleeding that won’t stop). While these are general warfarin warnings, diarrhea-related changes to INR can make such events more likely. ^[15]

Practical steps if diarrhea occurs

• Call your prescriber to report symptoms and ask whether an earlier INR check is needed. ^{[8] [9]}
• Review any new medications (including antibiotics, antifungals, and over‑the‑counter/herbal products) with your clinician or pharmacist, since these can interact with warfarin; closer INR monitoring is advised when starting or stopping them. ^{[10] [11]}
• Maintain hydration and a steady diet as tolerated to limit abrupt changes in vitamin K intake, which can affect INR; if you can’t maintain intake, let your clinician know. ^{[6] [7]}

Summary table

Topic	Key point	Why it matters for warfarin
Diarrhea as side effect	Listed in official labeling as a GI adverse reaction	Confirms it’s a recognized effect on warfarin. [1] [2]
Diarrhea + INR	Diarrhea and poor nutritional state can raise INR	Higher INR increases bleeding risk. [6] [7]
When to call	Diarrhea, fever, infection; severe or >24h; inability to eat	Prompts INR checks and management changes. [3] [4] [14]
Antibiotics interplay	Start/stop antibiotics → monitor INR closely	Drug interactions may shift anticoagulation level. [10] [11]
High‑risk antibiotics	TMP‑SMX and fluconazole linked to more GI bleeding on warfarin	May require alternatives or intensified monitoring. [12] [13]

Bottom line

• Diarrhea is a possible side effect of warfarin and, more importantly, can change your INR and increase bleeding risk. ^{[1] [2] [6] [7]}
• If you develop diarrhea especially if it is severe, lasts more than a day, or comes with fever or poor intake contact your healthcare provider to discuss whether you need an earlier INR check and any dosing adjustments. ^{[3] [4] [14]}