Lipitor (atorvastatin) is a medication used to lower cholesterol levels, while warfarin is a blood thinner used to prevent blood clots. According to the sources provided, there is a potential interaction between these two drugs that could affect the effectiveness of warfarin.

The drug interaction checker on Drugs.com [3] notes that taking Lipitor with warfarin may increase the risk of bleeding, and it is recommended to monitor patients closely for signs of bleeding. The website also recommends that patients should inform their doctor if they experience any unusual bleeding or bruising while taking these medications together.

A study published in the Journal of Hematology and Oncology [2] found that there is a potential for drug-drug interactions between statins, such as Lipitor, and warfarin. The study suggests that this interaction may be due to the effect of statins on the liver enzymes responsible for metabolizing warfarin, which could lead to changes in the blood levels of warfarin and affect its effectiveness.

Additionally, Drug Patent Watch [1] notes that Lipitor is a moderate inhibitor of the enzyme CYP2C9, which is responsible for metabolizing warfarin. Therefore, taking Lipitor with warfarin may lead to increased blood levels of warfarin, potentially increasing the risk of bleeding.

In conclusion, based on the sources provided, Lipitor may affect the effectiveness of warfarin. Patients taking these medications together should be closely monitored for signs of bleeding, and any unusual bleeding or bruising should be reported to a doctor immediately. It is important for patients to inform their doctor of all medications they are taking to avoid potential drug interactions.

Sources:
[1] https://www.drugpatentwatch.com/p/tradename/LIPITOR
[2] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8861410/
[3] https://www.drugs.com/drug-interactions/lipitor-with-warfarin-276-128-2311-0.html