According to the sources provided, monitoring liver function while taking Lipitor (atorvastatin) is important due to the potential risk of liver damage. The FDA recommends that liver function tests should be performed before starting Lipitor and then periodically thereafter to monitor for any adverse effects [3]. The frequency of monitoring should be determined by the healthcare provider based on the patient's individual needs and risk factors [2].

The drug patent watch website provides information on the Lipitor label, which recommends monitoring liver function tests at the beginning of therapy and as clinically indicated thereafter [1]. The label also advises that if elevated serum transaminases levels occur, treatment should be discontinued.

A review of the medical textbook, "Statins: Pharmacology, Clinical Uses, and Adverse Effects," confirms that monitoring liver function is necessary for patients taking statins, including Lipitor. The authors recommend that liver function tests should be performed at baseline and then repeated after 12 weeks of therapy and then annually thereafter [2].

In summary, it is recommended that liver function tests should be performed before starting Lipitor and periodically thereafter. The frequency of monitoring should be determined by the healthcare provider based on the patient's individual needs and risk factors. If elevated serum transaminases levels occur, treatment should be discontinued.

Sources:
[1] https://www.drugpatentwatch.com/p/tradename/LIPITOR
[2] https://www.ncbi.nlm.nih.gov/books/NBK430779/
[3] https://www.fda.gov/drugs/drug-safety-and-availability/fda-drug-safety-communication-important-safety-label-changes-cholesterol-lowering-statin-drugs