NEW YORK (CNNMoney.com) - The monster-sized market for anti-psychotic drugs is expected to keep growing for years, though at a slower pace.

"As drug classes go, growth rates in anti-psychotics are actually quite healthy," said Bruce Cranna, analyst for Leerink Swann. "But it's fairly well penetrated. It gets harder to grow as [the industry] gets bigger."

Sales for anti-psychotics, which are used to treat mental disorders like schizophrenia and mania, are slowing down in the U.S., but analysts say there's still room for them to grow in Europe, where the market is less mature.

Going forward, the $12 billion worldwide market for anti-psychotics will keep growing at 10 percent or slightly more, but that's down from 15 to 20 percent in 2003, Cranna said. But other analysts say growth could slow much further, to the 2 to 5 percent range.

The market is dominated by Zyprexa, Eli & Lilly's (up $0.09 to $54.59, Research) top-selling drug with $4 billion in 2005 sales. But worldwide sales for Zyprexa slipped 5 percent last year, primarily because of weak U.S. sales.

The industry-wide growth rate is currently being driven by Zyprexa's fast-growing competitors. But analysts do not expect the rapid sales growth for these other drugs to last.

For instance, Johnson & Johnson's (down $0.15 to $57.98, Research) Risperdal sales totaled $3.5 billion in 2005, and surged 21 percent in the first quarter of 2006, to $1.2 billion. That sales increase was driven by Risperdal Consta, an injectable form of the drug.

Astrazeneca's (up $0.68 to $52.81, Research) Seroquel, which grew by a third in 2005 to sales of $2.8 billion, is the fastest growing anti-psychotic, and is particularly popular in the expanding elderly population.

Pfizer's (up $0.02 to $24.95, Research) Geodon grew by 26 percent to $589 million in 2005 sales, while sales of Abilify, from Bristol-Myers (down $0.32 to $24.51, Research) and the Japanese drug maker Otsuka, jumped 54 percent to $912 million in 2005. (See correction).

But American sales for anti-psychotics may have reached a saturation point, and product liability lawsuits against the drugs have also hampered their growth. Analyst Jon LeCroy of Natexis Bleichroeder said the industry's growth is driven mainly by sales in Europe, a newer market where patients are less likely to sue than they are in America.

"[Europe] doesn't have the same legal issues we have here," said LeCroy. "In the U.S., there's the fear of being sued."

Lawsuits are fueled by health risks, and the sector is no stranger to them, or at least to warnings of them. LeCroy said that Pfizer's 2001 market entry for Geodon was slowed by a "black box" label, the most serious type of warning from the Food and Drug Administration, about health risks to the heart. However, the concerns were unfounded, he said.

"The problem never materialized," said LeCroy, noting the recent growth in Geodon sales. "Over the years, doctors have gotten more comfortable with Geodon."

But Zyprexa and other anti-psychotics have been blamed for causing diabetes, and their makers have been sued by former patients. In 2005, Lilly settled 8,000 Zyprexa-related lawsuits for $690 million. Astrazeneca currently faces about 60 lawsuits related to Seroquel brought by patients who blame the drug for their diabetes, and that number is expected to grow.

Barbara Ryan of Deutsche Bank North America said that drug sectors typically need a new product in order to drive growth.

But that won't happen until mid-2007, when Wyeth is expected to launch bifeprunox, an anti-psychotic that has not yet been filed with the Food and Drug Administration. Pfizer is currently in late-stage testing for asenapine, an experimental anti-psychotic, but is behind Wyeth (down $0.12 to $45.93, Research) in trying to get a new drug on the market.

LeCroy of Natexis Bleichroeder said that bifeprunox, if it's approved by the FDA, could achieve $400 million in U.S. sales by 2010, but that growth would quickly taper off.

"[Bifeprunox] will have early rapid growth," said LeCroy. "But it's a mature market, so things tend to stabilize a little faster."

But bifeprunox simply isn't different enough from existing anti-psychotics to truly shake up the market, said Deutsche Bank's Ryan. "We don't expect it to be all that different from what's out there," she said.

To read about an impending shake-up in the antidepressant industry, click here.

(Correction: An earlier version of this story misidentified one of the producers of Abilify. Return to story).