Lipitor is a brand name for atorvastatin, which is a medication used to lower cholesterol levels in the blood. According to the sources provided, liver function tests are recommended before starting atorvastatin and periodically thereafter to monitor for potential liver damage. The frequency of liver function monitoring may depend on several factors, including the patient's age, medical history, and other medications they may be taking.

The first source [1] states that liver function tests should be performed before starting atorvastatin and periodically thereafter. However, it does not specify the exact frequency of monitoring.

The second source [2] suggests that liver function tests should be performed before starting atorvastatin and repeated at six and 12 weeks after starting treatment. After that, liver function tests should be performed periodically, depending on the patient's risk factors for liver disease and the dose of atorvastatin they are taking.

The third source [3] recommends liver function tests before starting atorvastatin and then again within the first three months of treatment. After that, liver function tests should be performed periodically, depending on the patient's risk factors for liver disease and the dose of atorvastatin they are taking.

In summary, all three sources agree that liver function tests should be performed before starting atorvastatin and periodically thereafter to monitor for potential liver damage. The frequency of monitoring may depend on several factors, including the patient's age, medical history, and other medications they may be taking. It is important to follow the advice of a healthcare professional regarding the appropriate frequency of liver function monitoring while taking atorvastatin.

Sources:
[1] https://www.drugpatentwatch.com/p/tradename/LIPITOR
[2] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5325661/
[3] https://www.nhs.uk/conditions/statins/considerations/