Warfarin, Trimethoprim-Sulfamethoxazole, and an INR of 5.6

Mechanism

Trimethoprim-sulfamethoxazole (TMP-SMX) interacts with warfarin through several routes:

1. CYP2C9 inhibition. Sulfamethoxazole is a potent inhibitor of CYP2C9, the main enzyme metabolizing S-warfarin, the more potent enantiomer. This reduces warfarin clearance and raises INR.
2. Protein-binding displacement. Sulfonamides can displace warfarin from albumin, transiently increasing free drug.
3. Reduced vitamin K synthesis. Broad-spectrum antibiotics suppress gut flora that produce vitamin K, an effect that matters more when intake is poor.

The timing fits: the INR was stable at 2.4 two weeks ago and is 5.6 on day 5 of a seven-day course. That is a typical onset for a CYP2C9-mediated interaction.

Other contributors

• Amiodarone also inhibits CYP2C9 and has a very long half-life; it may already have increased warfarin sensitivity, and any new inhibitor acts on top of that.
• Acute infection can reduce oral intake of vitamin K–containing foods.
• Renal impairment (eGFR 38) affects clearance of some drugs and is a bleeding-risk marker, though warfarin itself is not renally cleared.
• Age increases bleeding risk independent of INR.

Causality and severity

Using a standard causality framework, the reaction is probable: there is a plausible temporal relationship, a known mechanism, and improvement is expected after withdrawal. Alternative causes such as liver disease, alcohol use, missed warfarin doses, or a new nonprescribed medicine should still be asked about. The severity is moderate: there is minor mucosal bleeding and a bruise, but no major bleeding, no hemodynamic compromise, and no neurologic symptoms.

Immediate actions

1. Stop warfarin temporarily. This is the first step for an INR above the therapeutic range with minor bleeding.
2. Assess for major bleeding. Ask about melena, hematuria, vomiting blood, severe headache, confusion, and abdominal or back pain. Check blood pressure and mental status. If major bleeding is suspected, arrange emergency transfer immediately.
3. Give oral vitamin K 1–2 mg. She has minor mucosal bleeding and multiple bleeding-risk factors (age 78, renal impairment, INR 5.6). Oral vitamin K is appropriate to lower the INR over the next 24 hours. I would not give intravenous vitamin K outside a monitored setting because of the risk of anaphylactoid reactions and overcorrection.
4. Do not give fresh frozen plasma or prothrombin complex concentrate for an INR of 5.6 without major bleeding. These are reserved for serious bleeding or urgent surgery.
5. Check CBC and renal function to quantify blood loss and review the eGFR. Her Hb has fallen only slightly, but a repeat count is prudent.
6. Repeat the INR in 24 hours, and again at 48–72 hours, because the effect of TMP-SMX and amiodarone may persist after the antibiotic stops.

Restarting warfarin

Restart at a reduced dose, such as 2–3 mg daily, once the INR is below 3.0 and bleeding has stopped, after discussing with the prescriber. The previous 4 mg dose is no longer appropriate while amiodarone and recent TMP-SMX exposure are in play. Arrange an INR check 3–5 days after restarting and weekly until stable.

Safety-net advice

She lives alone and has no transport, so I would:

• Give written instructions and a phone number.
• Ask her to seek urgent care for any new bleeding, black stools, severe headache, or confusion.
• Arrange a follow-up call or a home visit through a family member or community service.

Why not simply observe?

An INR of 5.6 with bleeding is not a 'wait and see' situation. The risk of a major bleed is real, and the intervention-holding warfarin and giving low-dose oral vitamin K-is low-risk and proportionate.

Alternative antibiotic

I would contact the prescriber with the INR result and ask for a change based on the urine culture and susceptibility. Reasonable options depend on susceptibility:

• Fosfomycin trometamol 3 g as a single dose is attractive because it has no clinically important CYP interaction with warfarin and is convenient.
• Nitrofurantoin can be used with caution for a short course in mild renal impairment, but it is less effective at an eGFR of 38 and should be avoided below 30 mL/min/1.73 m². It does not inhibit CYP2C9 to the same degree.
• Avoid fluoroquinolones where possible: they can potentiate warfarin and carry their own risks in older adults.
• Avoid trimethoprim-sulfamethoxazole for the remainder of this course unless there is no alternative and the INR can be monitored closely.

I would not choose an antibiotic solely by convenience; culture results and local resistance patterns matter.

Prevention plan

1. Flag her record as warfarin plus amiodarone, with a high interaction risk.
2. Advise her to tell every prescriber and pharmacist that she takes warfarin, and to avoid starting any new medicine-including herbal products and NSAIDs-without checking.
3. Check INR 3–5 days after starting any new antibiotic, even if she feels well.
4. Avoid NSAIDs and aspirin for pain; use paracetamol at the lowest effective dose and within label limits.
5. Keep vitamin K intake consistent rather than eliminating green vegetables.
6. Use a warfarin diary and a pill organizer, and involve a family member in appointments.

Counseling

Explain in plain language: 'Your blood is currently thinner than we want. The antibiotic you took for the urine infection made your warfarin stronger. We will pause the warfarin, give you a small dose of vitamin K, and check your blood again tomorrow.' Emphasize that she should not restart warfarin on her own, and that she must report bleeding that does not stop, black stools, or a severe headache. Give the advice in Arabic and confirm understanding with teach-back.