Lipitor is a commonly prescribed medication used to treat high cholesterol levels in the blood. It belongs to a class of drugs called statins, which work by blocking an enzyme in the liver that produces cholesterol. Although Lipitor is generally considered safe and effective, it can cause liver damage in some people. Therefore, it is important to monitor liver function regularly while taking this medication.

According to the National Health Service (NHS) [3], people taking Lipitor or any other statin should have a liver function test before starting treatment, again at 3 months, and then annually thereafter. This is to ensure that the medication is not causing any damage to the liver. However, the frequency of liver function tests may vary depending on the individual's medical history and other risk factors. For example, people who have pre-existing liver disease or who consume large amounts of alcohol may need more frequent monitoring.

The drug patent watch [1] also recommends regular liver function tests for patients taking Lipitor. However, the exact frequency of testing is not specified.

The book "Statins: Pharmacology, Clinical Use, and Adverse Effects" [2] states that liver function tests should be performed before starting statin therapy and periodically thereafter. The book suggests that testing frequency may vary depending on the patient's risk factors and the dose of the medication.

In conclusion, the recommended frequency for liver function tests while taking Lipitor is at least once a year, according to the NHS [3]. However, the frequency may vary depending on the individual's medical history and other risk factors. It is important to discuss any concerns or questions about monitoring liver function with a healthcare provider.

Sources:
[1] https://www.drugpatentwatch.com/p/tradename/LIPITOR
[2] https://www.ncbi.nlm.nih.gov/books/NBK548236/
[3] https://www.nhs.uk/conditions/statins/considerations/