After the patent for Lipitor, a popular cholesterol-lowering drug, expired in November 2011 [1], the drug faced generic competition. This competition led to a significant change in Lipitor's market share.

Before the patent expiration, Lipitor, manufactured by Pfizer, held a dominant position in the statin drug market, generating approximately $10.7 billion in sales in 2011 [1]. However, with the entry of generic versions, Lipitor's market share began to decline.

According to DrugPatentWatch.com, the loss of exclusivity for Lipitor resulted in a rapid decrease in sales [2]. By 2013, just two years after the patent expiration, Lipitor's sales had dropped to $2.3 billion, a decline of over 78% from its 2011 figures [2].

The decline in Lipitor's market share can be attributed to several factors. First, the availability of generic versions significantly reduced the cost of the drug for consumers. Second, insurance companies began to favor generic drugs over brand-name drugs due to their lower cost [3].

Moreover, the introduction of additional generic competitors further eroded Lipitor's market share. As of 2021, there are at least 15 approved generic versions of Lipitor available in the U.S. market [4].

In conclusion, the patent expiration of Lipitor led to a significant change in its market share due to the introduction of generic competitors. This resulted in a dramatic decline in sales, from $10.7 billion in 2011 to $2.3 billion in 2013.

Sources:
[1] "Lipitor (Atorvastatin Calcium) Tablets, for Oral Use." Pfizer. Accessed April 12, 2021. https://www.accessdata.fda.gov/drugsatfda_docs/label/2011/021028s053,021152s012,021153s012,021174s006lbl.pdf
[2] "Lipitor (Atorvastatin) Drug Patent Expiration." DrugPatentWatch. Accessed April 12, 2021. https://www.drugpatentwatch.com/drugs/lipitor
[3] "Generic Drugs: Lower Costs, Same Quality." FDA. Accessed April 12, 2021. https://www.fda.gov/drugs/generic-drugs/generic-drugs-lower-costs-same-quality
[4] "Atorvastatin Calcium Tablets." DailyMed. Accessed April 12, 2021. https://dailymed.nlm.nih.gov/dailymed/search.cfm?labeltype=all&query=atorvastatin+calcium+tablets