According to a report by StatNews, Pfizer's patent for Lipitor, the world's best-selling drug at the time, expired in 2011 [1]. Following this expiration, Lipitor's revenue experienced significant changes.

Pre-expiration, Lipitor was a massive commercial success, generating over $125 billion in sales from 1997 to 2011. However, after patent expiration, the drug's revenue drastically declined. A report by Express Scripts found that Lipitor's revenue dropped by 93% from 2011 to 2015, with the drug generating only $10.4 million in sales in 2015 [2].

The decline in Lipitor's revenue is largely attributed to generic competition. After patent expiration, multiple generic versions of the drug entered the market, significantly undercutting Pfizer's branded product [3]. The availability of affordable generic alternatives made it difficult for Pfizer to maintain market share.

Despite the decline in Lipitor's sales, Pfizer has continued to sell the drug and its various generic versions. However, the company's reliance on the brand has shifted, and it has since focused on developing new, high-margin products.

Sources:

[1] https://www.statnews.com/pharmalot/2016/11/01/pfizer-lipitor-generic-competition/
[2] https://www.expressscripts.com/make-the-switch/2016/05/31/express-scripts-2015-trends-report
[3] https://www.drugpatentwatch.com/DrugPatentExpirations/Atorvastatin