The drop in Lipitor's market share can be attributed to several factors, most notably the entry of generic alternatives into the market.

Lipitor, a cholesterol-lowering drug manufactured by Pfizer, was the best-selling pharmaceutical product in the world before the expiration of its patent [1]. However, with the patent's expiration in 2011, generic versions of the drug, such as atorvastatin, became available [1]. Consequently, this led to a significant decrease in Lipitor's market share.

According to DrugPatentWatch.com, the loss of exclusivity for Pfizer's Lipitor resulted in a decline in sales from $10.7 billion in 2011 to $2.1 billion in 2015 [2]. The website further notes that the introduction of generic alternatives resulted in a significant decrease in Lipitor's price, making it less profitable for pharmacies and healthcare providers [2].

Moreover, the Affordable Care Act (ACA) also played a role in Lipitor's market share drop. The ACA encouraged the use of generic drugs to reduce healthcare costs [3]. Consequently, healthcare providers were more likely to prescribe generic alternatives to Lipitor, further contributing to its decline in market share.

In summary, the expiration of Lipitor's patent, the introduction of generic alternatives, and the Affordable Care Act's emphasis on cost-effective healthcare contributed to the drop in Lipitor's market share.

Sources:

1. "Lipitor (atorvastatin) - Drug Uses, Side Effects, Interactions, Pictures, Warnings & Dosing." RxList, <https://www.rxlist.com/lipitor-drug.htm>.
2. "Lipitor (Atorvastatin) Patent Expiration, Generic Launch Dates, and Competitor Products." DrugPatentWatch, <https://www.drugpatentwatch.com/drugs/lipitor>.
3. "Generic Drugs and the Affordable Care Act." FDA, <https://www.fda.gov/drugs/generic-drugs/generic-drugs-and-affordable-care-act>.