After reviewing the provided sources, it is important to note that Lipitor (atorvastatin) and warfarin have the potential to interact with each other. Lipitor is used to lower cholesterol levels, while warfarin is a blood thinner used to prevent blood clots. It is essential to monitor the dosage of these drugs when taken together to avoid any potential adverse effects.

The drug patent for Lipitor expired in 2011 [1]. The FDA provides a prescribing information document for Lipitor, which includes a section on drug interactions. The document notes that warfarin can increase the risk of bleeding and that Lipitor can enhance the effects of warfarin, leading to an increased risk of bleeding. The prescribing information advises that patients taking Lipitor and warfarin together should have their prothrombin time or international normalized ratio (INR) monitored regularly [2].

The COVID-19 Treatment Guidelines issued by the National Institutes of Health also note that the use of Lipitor and warfarin together can increase the risk of bleeding. The guidelines advise that INR monitoring should be performed frequently when these drugs are used together [3].

There is no specific information provided in the sources about how often the dosage of Lipitor should be adjusted when taken with warfarin. However, it is clear that regular monitoring of INR is necessary to avoid any potential adverse effects.

In conclusion, Lipitor and warfarin have the potential to interact with each other, leading to an increased risk of bleeding. Patients taking these drugs together should have their INR monitored regularly to ensure that the dosage is appropriate and to avoid any potential adverse effects.

Sources:
[1] https://www.drugpatentwatch.com/p/tradename/LIPITOR
[2] https://www.fda.gov/media/155050/download
[3] https://www.covid19treatmentguidelines.nih.gov/therapies/antivirals-including-antibody-products/ritonavir-boosted-nirmatrelvir--paxlovid-/paxlovid-drug-drug-interactions/