Lipitor, also known as atorvastatin, is a medication used to lower cholesterol levels and prevent cardiovascular diseases [1]. According to a study published by the National Center for Biotechnology Information, atorvastatin is effective in reducing low-density lipoprotein (LDL) cholesterol levels, which is the primary cause of cardiovascular diseases [2].

The effectiveness of Lipitor is assessed periodically through clinical trials. A clinical trial registered with the National Institutes of Health is currently underway to evaluate the effectiveness of atorvastatin in reducing the risk of cardiovascular events in patients with type 2 diabetes mellitus [3].

In addition, Lipitor's patent expired in 2011, which means that generic versions of the drug are available on the market. These generic versions have been shown to be as effective as the brand-name drug [1].

Overall, Lipitor has been proven to be an effective medication in reducing cholesterol levels and preventing cardiovascular diseases. Its effectiveness is assessed periodically through clinical trials, and generic versions of the drug have been shown to be equally effective as the brand-name drug.

Sources:
[1] https://www.drugpatentwatch.com/p/tradename/LIPITOR
[2] https://www.ncbi.nlm.nih.gov/books/NBK430779/
[3] https://clinicalstudies.info.nih.gov/protocoldetails.aspx?id=06-H-0072&&query=