The expiration of Lipitor's patent had a significant impact on its revenue. Lipitor, a cholesterol-lowering drug produced by Pfizer, was the best-selling drug in the world before its patent expired in November 2011 [1].

Prior to the patent expiration, Lipitor generated approximately $13 billion in annual sales for Pfizer [2]. However, after the patent expired, generic versions of the drug, such as atorvastatin, entered the market, leading to a sharp decline in Lipitor's revenue.

According to a report by DrugPatentWatch.com, the loss of exclusivity for Lipitor resulted in a decline of over 90% in sales within the first year [3]. This decline can be attributed to the fact that generic drugs are typically sold at significantly lower prices than their brand-name counterparts [4].

The impact of Lipitor's patent expiration on Pfizer's revenue was substantial. In 2012, the year following the patent expiration, Pfizer's revenue declined by 17% compared to the previous year [5]. The decline in Lipitor's revenue was a major contributor to this decrease.

In conclusion, the expiration of Lipitor's patent had a significant negative impact on its revenue, leading to a sharp decline in sales and a substantial loss in revenue for Pfizer.

Sources:
[1] <https://www.reuters.com/article/us-pfizer-lipitor-patent-idUSBRE8AO0Y320121123>
[2] <https://www.fiercepharma.com/special-report/top-15-best-selling-drugs-2011>
[3] <https://www.drugpatentwatch.com/drugs/lipitor>
[4] <https://www.fda.gov/drugs/generic-drugs/what-are-generic-drugs>
[5] <https://www.reuters.com/article/us-pfizer-results-idUSBRE82Q0JZ20120327>