After the patent expiry of Lipitor (atorvastatin), a popular cholesterol-lowering drug, its revenue experienced a significant change. In 2011, Lipitor generated around $10.8 billion in revenue for Pfizer, accounting for approximately 20% of Pfizer's total revenue [1]. However, after the patent expiry in November 2011, generic versions of the drug entered the market, leading to a decline in Lipitor's sales.

By 2013, Lipitor's revenue had dropped to $2.3 billion, a decline of about 78.7% from its 2011 revenue [2]. The introduction of generic atorvastatin resulted in significant price reductions, making it more accessible to patients and reducing Lipitor's market share. According to DrugPatentWatch.com, the loss of exclusivity for Lipitor allowed other manufacturers to produce and sell generic versions of the drug, intensifying competition and further impacting Lipitor's revenue [3].

In summary, Lipitor's revenue decreased from $10.8 billion in 2011 to $2.3 billion in 2013, following the patent expiry and the subsequent entry of generic competitors into the market.

Sources:
[1] "Pfizer Reports Fourth-Quarter and Full-Year 2011 Financial Results." Pfizer, Inc., 26 Jan. 2012, www.pfizer.com/news/press-release/press-release-detail/pfizer_reports_fourth-quarter_and_full-year_2011_financial_results.
[2] "Pfizer Reports Fourth-Quarter and Full-Year 2013 Financial Results." Pfizer, Inc., 30 Jan. 2014, www.pfizer.com/news/press-release/press-release-detail/pfizer_reports_fourth-quarter_and_full-year_2013_financial_results.
[3] "Lipitor (Atorvastatin) Patent Expiry." DrugPatentWatch.com, www.drugpatentwatch.com/patent/lipitor-atorvastatin.