Warfarin Prescription: What You Need to Know

Warfarin is a blood thinner that doctors use to stop clots from forming. If you or someone you know has been told to take it, the first thing to understand is that the right dose is personal. One size doesn’t fit all, and the goal is to keep blood thin enough to prevent clots but not so thin that it bleeds.

Getting a Prescription and Finding the Right Dose

In Canada you need a doctor’s order to start warfarin. Most doctors will ask about your medical history, current meds, and any recent surgeries. After the first prescription, the dose is usually low – often 2 to 5 mg a day – and then gets tweaked based on blood tests.

The key test is the INR (International Normalized Ratio). It measures how long it takes your blood to clot. A typical therapeutic range is 2.0–3.0 for most conditions, but some heart‑valve patients need 2.5–3.5. Your doctor will schedule the first INR check within a few days of starting the drug and then every few days until the numbers settle.

If the INR is too low, the doctor may raise the dose by a small amount. If it’s too high, the dose is cut back or a short‑term “break” is given. You’ll hear the phrase “stay within the therapeutic window” a lot – it just means keeping the INR where it’s effective without risking bleeding.

Monitoring, Interactions, and Everyday Safety

Warfarin interacts with a long list of foods and medicines. Vitamin K‑rich foods like spinach, kale, and broccoli can lower the INR, while alcohol and some antibiotics can push it up. The trick is consistency: eat similar amounts of vitamin K each day and tell your pharmacist about every medication, including over‑the‑counter pills and herbal supplements.

Common drugs that can affect warfarin include aspirin, ibuprofen, some antidepressants, and antibiotics like ciprofloxacin. Always double‑check with a healthcare professional before adding a new drug. If you notice bruises, dark urine, or unusually heavy bleeding, call your doctor right away – those could be signs the INR is too high.

Most pharmacies in Canada offer point‑of‑care INR testing. Some even let you bring a home test kit and report the results online. Keeping a small log of your INR, dose changes, and any new meds helps your doctor make quick adjustments.

When you travel, bring a copy of your prescription and a recent INR report. Not all countries have the same monitoring standards, so it’s smart to know where you can get a test abroad.

Finally, never stop warfarin abruptly. If you need to pause for a procedure, your doctor will give a clear plan to bridge you with another blood thinner if needed.

Warfarin can feel confusing at first, but with regular INR checks, a stable diet, and open communication with your healthcare team, it works well for many people. Keep the conversation going, track your numbers, and you’ll stay on the safe side while the medication does its job.