Coumadin (Warfarin) 1‑10 mg Colour-Coded Tablets: Prescription Vitamin K Antagonist Anticoagulant for Atrial Fibrillation, DVT/PE, and Mechanical Heart Valves

Information last reviewed: May 2026 — for educational purposes only.

Coumadin (warfarin sodium) is the oldest and most widely studied oral anticoagulant, used for more than 70 years to prevent and treat thromboembolic disease. It inhibits vitamin K epoxide reductase (VKOR), preventing the recycling of vitamin K — the essential cofactor required for the gamma-carboxylation and activation of clotting factors II (prothrombin), VII, IX, and X, as well as the anticoagulant proteins C and S. By blocking production of these factors, warfarin shifts the coagulation balance toward reduced clot formation capability. The effect takes 2–3 days to reach peak because existing circulating clotting factors must be depleted before the full anticoagulant effect is apparent; bridging with heparin is often used in high-clot-risk patients during initiation.

Warfarin is a narrow therapeutic index drug — the difference between an effective and a dangerous dose is small and varies considerably between patients due to genetic polymorphisms in CYP2C9 (metabolises warfarin) and VKORC1 (warfarin's target enzyme). Consequently, warfarin requires regular INR blood test monitoring to maintain the anticoagulation effect within the therapeutic range. Warfarin's numerous food and drug interactions add to the complexity of management and are a leading cause of medication errors.

What Is Warfarin?

Warfarin sodium is available in multiple strengths that are each a different colour — this colour coding allows patients and healthcare providers to identify doses visually, reducing dispensing errors particularly when doses are frequently adjusted. Warfarin is taken once daily at the same time each day. The INR (International Normalised Ratio) measures how long it takes blood to clot compared to a normal standard: an INR of 1.0 is normal (no anticoagulation); the therapeutic target for most indications is INR 2.0–3.0 (atrial fibrillation, DVT/PE treatment and prevention); for mechanical mitral valves or high-risk mechanical aortic valves, the target is typically INR 2.5–3.5. INR must be monitored at least monthly when stable, and more frequently when doses change, new medications are started, or INR drifts outside range.

Prescription Status

Warfarin is prescription-only in the United States and is strictly not available over the counter. It requires physician oversight for initiation, dose titration, and regular INR monitoring. For some indications such as non-valvular atrial fibrillation, newer DOACs (apixaban, rivaroxaban) are preferred over warfarin due to their more predictable pharmacology and lack of monitoring requirements — however, warfarin remains the anticoagulant of choice for mechanical heart valves (DOACs contraindicated), certain valvular AF, and in patients with severe renal impairment where DOACs are not appropriate.

Strengths and Available Forms

Warfarin tablets are available in 9 strengths, each with a specific colour to facilitate identification:

• 1 mg — pink
• 2 mg — lavender/purple
• 2.5 mg — green
• 3 mg — tan/beige
• 4 mg — blue
• 5 mg — peach/salmon
• 6 mg — teal
• 7.5 mg — yellow
• 10 mg — white/dye-free

Generic warfarin sodium tablets are available in the same strengths and colour coding. Patients should use the same manufacturer's tablets consistently — different generics may have minor formulation differences that can affect INR stability.

Price of Coumadin and Generic Warfarin

Generic warfarin sodium tablets are among the least expensive prescription medications available — typically a few dollars per month through discount programmes. Brand-name Coumadin costs significantly more. The major ongoing cost with warfarin therapy is INR monitoring — regular blood tests (initially weekly, eventually monthly when stable), which may involve lab draws, clinic visits, or home INR monitoring devices. Warfarin pharmacogenomic testing (CYP2C9/VKORC1) may reduce time to stable INR and is occasionally covered by insurance.

Frequently Asked Questions

Which foods interact with warfarin and how should patients manage diet?

Vitamin K in foods directly antagonises warfarin's mechanism — eating large or variable amounts of vitamin K-rich foods (leafy green vegetables: kale, spinach, broccoli, Brussels sprouts, parsley; also green tea and some vegetable oils) can significantly reduce the INR. Conversely, grapefruit and grapefruit juice inhibit CYP3A4 (partly involved in warfarin metabolism) and can increase INR. The critical advice is consistency rather than avoidance: patients should eat a consistent weekly diet and not dramatically change their intake of vitamin K-rich foods. Sudden increases cause INR to drop (clot risk); sudden decreases cause INR to rise (bleed risk). Patients should always inform their anticoagulant clinic of significant dietary changes.

What medications interact with warfarin?

Warfarin has more clinically significant drug interactions than almost any other medication. Major categories include: antibiotics (metronidazole, ciprofloxacin, azithromycin, fluconazole — all increase INR by inhibiting CYP2C9 or reducing vitamin K-producing gut bacteria); NSAIDs (aspirin, ibuprofen — increase bleeding risk by platelet inhibition, not INR change; gastric bleeding risk additive); amiodarone (potent CYP2C9 inhibitor — dramatically and persistently raises INR); SSRIs/SNRIs (modest INR elevation); rifampin (powerful CYP inducer — dramatically reduces INR). Any new prescription, OTC medication, herbal supplement (especially St John's Wort, which reduces INR significantly), or significant dietary change should prompt closer INR monitoring.

What are the reversal agents for warfarin in case of emergency?

For non-urgent reversal or supratherapeutic INR without major bleeding, oral or IV phytonadione (vitamin K) lowers the INR within 6–24 hours by replenishing the substrate for vitamin K-dependent clotting factor synthesis. For urgent reversal (major bleeding or emergency surgery), 4-factor prothrombin complex concentrate (4F-PCC) — containing factors II, VII, IX, and X — immediately replenishes clotting factors and reverses anticoagulation within minutes. Fresh frozen plasma (FFP) is an alternative when PCC is unavailable but requires larger volumes and longer infusion times. Recombinant factor VIIa is reserved for life-threatening bleeding refractory to other measures. Patients on warfarin should carry a wallet card stating their anticoagulation status and INR range for emergency providers.

← Back to Blood Thinner Medications