FRIDAY, Jan. 17, 2020 (HealthDay News) -- Patients taking the blood thinner warfarin have been told that it should be taken at night, but a new study found the time of day doesn't matter.

"Whether warfarin is taken in the morning, or the evening, its therapeutic effect is the same," said lead researcher Dr. Scott Garrison, an associate professor of family medicine at the University of Alberta in Edmonton, Canada.

"Health care providers should stop telling patients to use their warfarin in the evening; rather, warfarin should be taken whenever regular compliance would be easiest for patients," he added.

The drug is used to prevent blood clots that can cause strokes, heart attacks and blockages. It is also used to treat the abnormal heart rhythm called atrial fibrillation.

Doctors have had a practical reason for telling patients to take warfarin in the evening, Garrison said. Patients who use it must have blood tests every one to four weeks to determine if their dose is correct -- too much can lead to bleeding, too little won't prevent clots. Taking the drug at night meant less time between getting a test result and adjusting the dose.

"That is all the docs were really thinking about," Garrison said.

For the study, his team randomly assigned 217 patients to take their warfarin either in the morning or the evening.

Over seven months, the researchers tracked the amount of time patients' blood levels were outside the range where warfarin is most effective.

No significant difference in effectiveness was seen regardless of when the drug was taken, the study found.

Warfarin is a difficult drug to manage, said Dr. Salman Azhar, director of the stroke program at Lenox Hill Hospital in New York City. He was not involved in the study.

"It is difficult because it's a drug that is so variable depending on what you eat, and what other medications you're taking. For example, the drug can change by diet like eating dark leafy vegetables and broccoli, or drugs like antibiotics," he said.

These vegetables contain vitamin K, which promotes clotting. And antibiotics change the way warfarin is metabolized in the liver, Azhar said.

Azhar noted that other studies have found that patients using warfarin were in the therapeutic range only about 55% of the time. That means they were not getting a benefit from the drug up to about 40% of the time, he said.

The difficulty in managing warfarin is the main reason doctors are switching their patients to newer anticoagulants, Azhar said.

These newer drugs, such as Xarelto (rivaroxaban) and Eliquis (apixaban), don't require frequent blood tests or other monitoring and aren't affected by diet or other drugs, Azhar said.

"The biggest group of people who are getting anticoagulation are the people with atrial fibrillation, and almost all of them are eligible to get these new medications," he said, "And quite frankly, I think most of us are putting people on these new medications and not electing to go the route of warfarin."

Warfarin is still the drug of choice for patients who have mechanical heart valves or a higher genetic risk of chronic bleeding or, who tend to have more clotting not from a heart problem, just from something to do with their blood, Azhar said.

The report was published in the January issue of the Annals of Family Medicine.

More information

Learn more about warfarin from the American Heart Association.