Welcome to the patent cliff, the most influential feature of today's pharma landscape.

In the drug business, exclusivity is everything. Once a branded drug gets generic competition, pricing power fades and sales slowly drain away, leaving drugmakers dependent on the next new thing. It's all very reasonable in theory, but when the drugs losing patent protection are the biggest in the business--and they're all falling off patent around the same time--even the most rational companies can get a bit panicky.

Here, so many drugs are falling off patent that the path forward has twisted, and the entire industry is looking to find its way. Some companies, such as Pfizer and Merck, have merged with major rivals. Others have snapped up acquisitions in ancillary businesses, such as generics and over-the-counter medicines, and expanded into countries they previously ignored. Still others have forged ahead, betting that they'd manage to develop enough new products to make up the difference.

Although a few major drugmakers have already suffered their biggest drops off said cliff, most are bracing for the fall. Here are the 10 biggest blockbusters that have already lost or will lose exclusive market rights in the U.S. over the next 12 months. They include some of the best-selling drugs ever, such as Pfizer's Lipitor and Bristol-Myers Squibb and Sanofi's Plavix. We ranked them not by U.S. sales volume, but by their weight in each company's U.S. revenue stream.

1. Forest Laboratories - Lexapro
2. Takeda Pharmaceutical - Actos
3. Bristol-Myers Squibb - Plavix
4. AstraZeneca - Seroquel
5. Eli Lilly - Zyprexa
6. Pfizer - Lipitor
7. Merck - Singulair
8. Novartis - Diovan
9. Teva Pharmaceutical - Provigil
10. Abbott Laboratories - TriCor