Lipitor is a medication used to lower cholesterol levels in the body. According to Drug Patent Watch [1], the active ingredient in Lipitor is atorvastatin, which is classified as a statin. Statins are known to have potential side effects, including liver damage. Therefore, it is essential to monitor liver function when taking Lipitor.

A study published in the National Center for Biotechnology Information [2] states that liver function tests should be performed before starting Lipitor therapy and then periodically thereafter. The frequency of liver monitoring may vary depending on the patient's age, medical history, and the dose of Lipitor prescribed. The study recommends that liver function tests should be performed at baseline, 12 weeks after starting Lipitor, and then every six months for the first year of treatment. After the first year, liver function tests can be performed annually or as needed.

UpToDate [3] also recommends monitoring liver function tests before starting Lipitor and then periodically thereafter. The frequency of monitoring should be based on the patient's risk factors, including age, medical history, and dose of Lipitor. The recommendation is to perform liver function tests at baseline, 12 weeks after starting Lipitor, and then every six months for the first year of treatment. After the first year, liver function tests can be performed annually or as needed.

In conclusion, both the National Center for Biotechnology Information and UpToDate recommend monitoring liver function tests before starting Lipitor and then periodically thereafter. The frequency of monitoring may vary depending on the patient's age, medical history, and the dose of Lipitor prescribed. The recommended frequency is to perform liver function tests at baseline, 12 weeks after starting Lipitor, and then every six months for the first year of treatment. After the first year, liver function tests can be performed annually or as needed.

Sources:
[1] https://www.drugpatentwatch.com/p/tradename/LIPITOR
[2] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4110821/
[3] https://www.uptodate.com/contents/statins-actions-side-effects-and-administration