Based on the available information, lipitor dose adjustments may be necessary when co-administered with warfarin.

Warfarin is an anticoagulant that works by inhibiting vitamin K-dependent coagulation factors, while Lipitor (atorvastatin) is a medication used to lower cholesterol levels. According to a study published in PubMed, the concomitant use of atorvastatin and warfarin may increase the international normalized ratio (INR), a measure of blood clotting time, indicating an increased risk of bleeding (source: <https://pubmed.ncbi.nlm.nih.gov/28357715/>).

The American Academy of Family Physicians also recommends monitoring coagulation parameters, such as INR, when initiating or changing the dose of atorvastatin in patients receiving warfarin therapy (source: <https://www.aafp.org/pubs/afp/issues/2007/0515/p1487.html>).

Therefore, it is crucial to monitor patients closely and adjust the warfarin dose as necessary when initiating or changing the dose of Lipitor. Healthcare providers should consider the potential drug-drug interaction and the patient's individual response to therapy.

Sources:

1. <https://www.drugpatentwatch.com/p/tradename/LIPITOR>
2. <https://pubmed.ncbi.nlm.nih.gov/28357715/>
3. <https://www.aafp.org/pubs/afp/issues/2007/0515/p1487.html>