Patent expirations for lucrative blockbuster drugs eroded third-quarter sales for top drugmakers Eli Lilly & Co. (NYSE: LLY), Bristol-Myers Squibb Co. (NYSE: BMY) and Merck & Co. Inc. (NYSE: MRK), as they faced competition from low-cost generic drugs.

Pfizer Inc. (NYSE: PFE) is set to report earnings for the past quarter on Oct. 31 and could report similar results.

Both Lilly and Bristol-Myers posted third-quarter earnings below analysts’ expectations, while Merck topped expectations with the help of its next blockbuster product -- the Januvia/Janumet franchise that together brought in nearly $1.4 billion in the quarter.

Drug companies must constantly bring new drugs to market to keep thriving.

Sales of Lilly’s antipsychotic drug Zyprexa, which lost patent exclusivity in October 2011, fell 68 percent to $375 million in the third quarter. The drug once brought in more than $5 billion annually. Lilly will also lose patents protecting the antidepressant Cymbalta and the insulin replacement drug Humalog next year.

Demand for Bristol-Myers’ blood clot preventer, Plavix, which had been one of the world’s best-selling drugs until its American patent lapsed in May, plunged 96 percent to $64 million.

Singulair, Merck's best-selling drug that treats asthma and allergies, lost U.S. patent protection in August and saw sales tumble 55 percent to $602 million for the third quarter. The drug is due to lose patent protection in major European countries in February.

Pfizer is expected to report a plunge in sales for its cholesterol fighter Lipitor, when the company reports next Wednesday.

Indianapolis-based Lilly said Wednesday it earned $1.33 billion, or $1.18 a share, compared with $1.24 billion, or $1.11 a share, a year earlier.

Excluding one-time items, it earned 79 cents a share. Analysts on average expected 83 cents, according to Thomson Reuters. Lilly’s world-wide revenue fell 11 percent to $5.44 billion, below Wall Street estimates of $5.62 billion.

New York-based Bristol-Myers reported a loss of $711 million, or 43 cents a share, in the third quarter, reversing profit of $969 million, or 56 cents a share, a year earlier. Excluding one-time items, Bristol-Myers earned 41 cents a share in the quarter. Analysts on average had expected 42 cents, according to Thomson Reuters. Sales fell 30 percent to $3.74 billion, 6.3 percent less than analyst projections.

Whitehouse Station, N.J.-based Merck said it earned $1.73 billion, or 56 cents per share, in the quarter, up from $1.69 billion, or 55 cents a share, a year earlier. Excluding charges totaling $1.2 billion, net income was $2.93 billion, or 95 cents a share, 2 cents more than analysts expected. Revenue, however, declined 4 percent to $11.5 billion. Analysts had expected $11.57 billion.

Shares of Merck dropped 11 cents to $46.19 in Friday midday trading. Pfizer shares were down 12 cents at  $25.49, Lilly was off 59 cents at $50.48, and Bristol Myers fell 14 cents to $33.59.