Lipitor, also known as atorvastatin, is a medication used to lower cholesterol levels in the blood. According to the NHS, it is recommended that liver function tests are carried out before starting treatment with Lipitor and then repeated as necessary. The frequency of these tests can vary depending on the patient's medical history and other medications they may be taking. It is important to note that liver function tests should be carried out regularly to monitor any potential side effects of Lipitor, such as liver damage. According to the Drug Patent Watch, Lipitor is typically prescribed in doses ranging from 10mg to 80mg, and the frequency of liver function tests should be determined by the prescribing doctor based on the patient's individual needs. The National Center for Biotechnology Information states that liver function tests should be carried out at the start of treatment with atorvastatin and then repeated every six months for the first year of treatment. After the first year, liver function tests should be carried out annually or as needed based on the patient's medical history. In conclusion, the frequency of liver function tests for patients taking Lipitor should be determined by their prescribing doctor based on their individual medical history and other medications they may be taking.

Sources:
[1] https://www.nhsinform.scot/tests-and-treatments/medicines-and-medical-aids/types-of-medicine/statins
[2] https://www.drugpatentwatch.com/p/tradename/LIPITOR
[3] https://www.ncbi.nlm.nih.gov/books/NBK548236/