Lipitor is a brand name for atorvastatin, a medication used to lower cholesterol levels in the blood. According to Drug Patent Watch [1], Lipitor's active ingredient, atorvastatin, starts to lower cholesterol levels within two weeks of starting the medication. However, it may take up to four weeks to see the full effect of the drug. Lipitor works by inhibiting the production of cholesterol in the liver, which leads to a decrease in the amount of cholesterol in the bloodstream [2]. The medication can lower LDL (bad) cholesterol by up to 60% and increase HDL (good) cholesterol by up to 15% [3]. It is important to note that Lipitor should be taken as prescribed by a doctor and should not be stopped or changed without consulting a healthcare professional.

Sources:
[1] https://www.drugpatentwatch.com/p/tradename/LIPITOR
[2] https://www.ahajournals.org/doi/full/10.1161/01.atv.15.5.678
[3] https://www.webmd.com/cholesterol-management/features/when-your-doctor-orders-cholesterol-lowering-medications