Based on the available information, yes, Lipitor (atorvastatin) can potentially affect Warfarin's efficacy. According to the drug interactions information from Drugs.com, when Lipitor and Warfarin are taken together, it can increase the International Normalized Ratio (INR) or prothrombin time (PT), which are measures of blood clotting time [3]. This means that Warfarin's anticoagulant effect is potentially enhanced, which could affect its efficacy.

The National Center for Biotechnology Information (NCBI) states that statins, such as Lipitor, can interact with Warfarin, and this interaction can result in an increased risk of bleeding [2]. This suggests that Warfarin's efficacy could be affected by Lipitor, as an increased risk of bleeding could indicate that the blood is not clotting as effectively as it should.

Therefore, patients taking both Lipitor and Warfarin should be closely monitored by their healthcare provider to ensure that their INR or PT remains within the therapeutic range [3]. Patients should also inform their healthcare provider of any changes in their medication, including the starting or stopping of new medications [1].

In summary, based on the available information, Lipitor can affect Warfarin's efficacy by increasing the risk of bleeding, which suggests that the blood is not clotting as effectively as it should. Patients taking both medications should be closely monitored by their healthcare provider.

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>