NEW YORK (CNN/Money) - Drugmakers rue the day when patents expire on blockbuster products, but some companies have found ways to relieve the pain of patent loss and wring more sales out of name brands.

When patents expire on name brand drugs, generic companies are quick to produce cheaper versions. Sales on a blockbuster drug can plunge 80 percent or more the first year after a generic competitor hits the market, according to Bernstein & Co. analyst Gbola Amusa.

Brand-name drugs can cost three times their generic equivalents, according to the Generic Pharmaceutical Association, even though the FDA insists there is no therapeutic difference between generics and name brands.

But to fight back, some companies, like GlaxoSmithKline, Eli Lilly and Co. and Schering-Plough Corp., have tweaked blockbuster drugs into new versions as patents on their original drugs expire.

As Glaxo (up $0.20 to $50.03, Research) lost its patent on Paxil, an antidepressant, the company developed a new version that patients take just once a day, rather than twice. As Lilly's (unchanged at $58.75, Research) patent expired on Prozac, another antidepressant, the company introduced Prozac Weekly. Schering-Plough (up $0.10 to $20.39, Research) launched Clarinex, a tweaked version of its blockbuster antihistimine the same year that Claritin, its blockbuster antihistamine, lost patent protection.

Schering-Plough also beat the generic companies at their own game by launching Claritin as a blockbuster over-the-counter drug within days of losing its patent. "The generics market got trashed when Claritin got authorized for OTC," said Rauch, who rates Schering-Plough and Lilly a buy.

Claritin was making about $3 billion in sales when it lost patent exclusivity in December 2002, said Rauch. The combined Claritin and Clarinex products take in about $1 billion in annual sales, or about a third of prescription Claritin sales at their peak.

So don't be surprised if Aventis Pharmaceuticals tries to launch an over-the-counter version of Allegra when the antihistamine's patent expires next year, Rauch said. Aventis spokeswoman Melissa Feldman said Allegra is "appropriately marketed" as a prescription drug and declined to discuss the drugmaker's plans.

GlaxoSmithKline lost its patent on Paxil in 2003. Sales of the former blockbuster totaled $172 million in 2004 in the U.S., about one-fifth the sales for the newly-available generic version, paroxetine, according to Drug Topics, an industry publication, and Verispan, a healthcare information provider.

GlaxoSmithKline has stayed in the game with Paxil CR, a once-a-day "controlled release" drug that had $824 million in U.S. sales last year. Even with Paxil CR, the company lost half its revenue to the generic market. But the daily capsule "tends to prevent overall erosion of the flagship product," said Amusa, who rates GlaxoSmithKline a buy.

The big advantagoe of once-a-day Paxil CR over twice-a-day generic is that "patients with depression don't always take their pills," said Amusa. "The generics can't do once-a-day but Glaxo can, and that permits a lot of patients to continue to be loyal to the branded product."

Paxil sales were still a small slice of GlaxoSmithKline's total sales of $39 billion last year, and could shrink this year. In March, U.S. marshals seized supplies of Paxil CR at plants in Tennessee and Puerto Rico, following the FDA's charge that the tablets tend to break apart.

GlaxoSmithKline agreed to fix the problems in April and Bernstein's Amusa projected the company will lose about $200 million in Paxil CR sales this year, assuming it gets the product back on the market by July 1.

Amusa said that the company could have a hard time winning back old customers. "The ones that are lost will probably be lost for good," said Amusa.

Prozac, a drug with strong product recognition, lost its patent in 2001. Eli Lilly made $182 million in U.S. sales of Prozac in 2004, less than a third of its generic equivalent, fluoxetine.

With the introduction of patented Prozac Weekly, the drug has managed to compete against once-a-day generics. But Barbara Ryan, an analyst for Deutsche Bank North America, refrained from calling it a success story.

"I wouldn't say [Prozac] is selling well," said Ryan, who rates Lilly a hold and Schering-Plough a buy. "It's nice that they preserved $182 million from the Prozac franchise, but it was $2 billion, so it's not real meaningful for Lilly."

So why don't all drugmakers launch new versions of every drug about to go off patent?

Because it doesn't always work, said Rauch. The A.G. Edwards analyst noted Merck tried to get approval for an over-the-counter version of Mevacor, the cholesterol-lowering drug, but the FDA turned it down because patients could not accurately read their own cholesterol levels.

None of the analysts interviewed for this story own stock in the companies discussed here and their firms have not conducted banking with them.