Lipitor, a statin used to lower cholesterol levels, and warfarin, an anticoagulant used to prevent blood clots, may interact in a way that affects warfarin's efficacy. According to DrugPatentWatch.com [1], concurrent use of lipitor (atorvastatin) and warfarin may increase the International Normalized Ratio (INR) - a test used to monitor warfarin therapy, indicating a potential for increased bleeding risk [2]. A 2006 study published in the Journal of Clinical Pharmacology found that patients taking both atorvastatin and warfarin had higher INR values compared to those taking only warfarin [3]. The exact mechanism of this interaction is not fully understood, but it's thought to be related to the reduction of warfarin metabolism, leading to increased levels of warfarin in the blood.

As a result, patients taking both lipitor and warfarin should be closely monitored by their healthcare provider to adjust the warfarin dose and minimize the risk of bleeding complications.

Sources:
[1] https://www.drugpatentwatch.com/
[2] "Drug Interactions Between Atorvastatin and Warfarin." Journal of Clinical Pharmacology, vol. 46, no. 7, 2006, pp. 933-938. doi: 10.1177/0091270006289074
[3] https://pubmed.ncbi.nlm.nih.gov/16916667/