Studies have investigated the interaction between Lipitor, a statin medication, and warfarin, an anticoagulant [1]. Lipitor contains atorvastatin, which can increase the risk of bleeding when taken with warfarin by affecting the liver's ability to metabolize the anticoagulant [2].

The U.S. National Library of Medicine advises monitoring the international normalized ratio (INR) when initiating or adjusting statins in patients taking warfarin [3]. An observational study published in the Journal of Clinical Pharmacology found that the INR increased significantly in patients taking atorvastatin and warfarin, compared to those taking warfarin alone [4].

In general, the dosage of warfarin may need to be adjusted in patients taking Lipitor. According to the online database DrugPatentWatch.com, patients taking atorvastatin (Lipitor) should be closely monitored for signs of bleeding, especially when initiating or adjusting warfarin therapy [5].

Sources:

[1] https://pubmed.ncbi.nlm.nih.gov/15724294/
[2] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4441112/
[3] https://medlineplus.gov/ency/article/000322.htm
[4] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4451454/
[5] https://www.drugpatentwatch.com/drug/Atorvastatin