Consuming small to moderate amounts of wine or other alcoholic beverages (1 to 2 drinks) is generally considered safe with Coumadin (warfarin) for those with normal liver function.
However, it does appear that consuming large amounts of alcohol in a short period of time, or chronic consumption, can adversely affect liver function.
This may inhibit warfarin metabolism as well as decrease liver-synthesized clotting factors, both of which can increase the risk of bleeding events.
What Do The Studies Say?
Several studies have investigated the effects of warfarin and blood clotting in general. Below is an overview of a few pertinent ones.
One large review study, published in the Annals of Internal Medicine, evaluated the effects of various food and drink on Coumadin (warfarin). They found that alcohol alone did not produce any negative interactions with warfarin. They list alcohol as 'Drugs with No Effect on Warfarin'.
It is important to note that the study did find 3 cases of hemorrhage in those combining alcohol in warfarin. However, in all 3 cases, the individuals had concomitant liver disease. The theorized mechanism behind this is that that any sort of liver dysfunction decreases the synthesis of vitamin K-dependent clotting factors. Alcohol can potentiate existing liver dysfunction, leading to an increase in bleeding risk.
Another study that specifically looked at wine consumption, published in JAMA Internal Medicine, evaluated the effects of consuming 296 mL of fortified wine (20% alcohol by volume) while fasting. This study did not find any interaction with Coumadin either:
The data showed no significant changes in the means either of the one-stage prothrombin time or of the plasma levels of warfarin of any subject during the daily ingestion of 296 mL of fortified wine during fasting
The study went on to conclude:
...the moderate use of wine during fasting or the liberal use during meals has no notable effect on long-term therapy with racemic warfarin in normal subjects.
A study published in Pharmacoepidemiology & Drug Safety discusses the significant risk of bleeding in those taking warfarin who also screened positive for moderate/severe alcohol misuse or heavy episodic drinking.
These individuals had nearly a two-fold increase in odds of a major bleeding event, specifically gastrointestinal bleeding. Similar to the other studies I have referenced, the authors of this one believe the increased risk of bleeding was due to alcohol-induced liver dysfunction. They did not report of any bleeding events in those with normal liver function.
Most studies report that mild to moderate consumption of alcohol has no significant effect on warfarin or bleeding activity.
However, it's evident that any interaction that exists is likely due to the fact that chronic (or heavy acute) consumption of alcohol can lead to liver disease and dysfunction. This may result in a decrease in warfarin metabolism (and a subsequent increase in concentrations) as well as a decrease in liver produced clotting factors.
So overall, a glass of wine at dinner should not have a significant effect on your warfarin dose and should not put you at risk for bleeding. Nevertheless, it would be prudent to avoid large amounts of alcohol and to have your INR/PT levels checked if your alcohol intake changes or you notice adverse effects.
Additionally, if you have liver disease or liver dysfunction, alcohol could exacerbate your condition, which in turn could significantly affect warfarin metabolism or your body's ability to produce blood-clotting factors. Therefore, be sure to follow your doctor's advice regarding diet and drinking as they know your complete medical situation.
Answer SummaryMost studies show that small amounts of alcohol do not have a significant effect on warfarin or blood clotting ability. However, chronic or heavy acute consumption can decrease metabolism of the drug and decrease liver-product clotting factors, increasing the risk of beeding events.
- Elsevier ClinicalKey: Warfarin (Accessed 1/17/19)
- Interactions of warfarin with drugs and food. PubMed (Accessed 1/17/19)
- Lack of effect of fortified wine ingested during fasting and anticoagulant therapy. PubMed
- Enhanced antithrombotic effect of warfarin associated with low-dose alcohol consumption. PubMed
- Alcohol misuse, genetics, and major bleeding among warfarin therapy patients in a community setting. PubMed