LINDA A. JOHNSON AP Business Writer TRENTON, N.J. (AP) — Growth of prescription drug revenue worldwide will likely dip to between 4 percent and 6 percent next year — the lowest rate in at least a decade, according to a forecast released Thursday. IMS Health, which tracks sales of prescription drugs in 220 countries, cited increased sales of generic drugs, the global recession and other factors. In the United States, growth is expected to be about 5 percent, an improvement from last year, according to a five-year forecast from IMS. But that's up from the research firm's estimates from just several months ago, and the new estimates indicate global prescription drug sales could hit $1 trillion per year in 2013. One-fifth of all pharmaceutical sales growth will come from China between now and then, with the pharmaceutical market there growing at more than 20 percent each year, according to IMS. By 2013, the world's most populous country should become its third-biggest prescription drug market — after the U.S. and No. 2 Japan — with roughly $80 billion in 2013 sales. China was No. 10 just six years ago. IMS projects worldwide sales growth of 4 percent to 7 percent each year through 2013. "We are seeing the economic downturn influencing both patient behavior and payer strategies," said Murray Aitken, senior vice president of the company's Healthcare Insight analysis division. He said patient spending is especially constrained in countries such as Russia, Mexico and South Korea where people directly pay for much of their healthcare. There has been less of an effect on sales in countries with national health care plans, such as Japan, Germany and Spain, he said. IMS projects anemic U.S. prescription drug revenue growth of 4.5 percent to 5.5 percent this year, 3 percent to 5 percent next year and average growth of just 2 percent to 5 percent a year from 2008 through 2013. That's still better than last year, when U.S. sales edged up 1.5 percent — one-sixth the growth rate just two years before. IMS expects U.S. drug sales to hit $325 billion to $355 billion in 2013, just over a third of the global total. Revenue worldwide could be affected by factors such as the severity of the swine flu pandemic and what happens with U.S. health care reform legislation, Aitkin noted. A major cause of the anticipated slower worldwide growth is competition from generic drugs launched in recent years. Some blockbuster drugs already have lost patent protection and seen their sales plummet as generic rivals hit pharmacy shelves. That trend will increase over the next five years. "During that period, we have an unprecedented $137 billion worth of products losing patent protection, particularly in 2011 and 2012," Aitken said. New blockbuster products — those with sales topping $1 billion a year — will only be able to replace some of the revenue prescription drug companies will lose, he said. To compensate, big drugmakers increasingly are targeting emerging markets, countries with big populations and a growing middle class now spending more on health care. As a result, IMS projects average annual sales growth of 13 percent to 16 percent through 2013 in the top emerging markets: China plus Brazil, Russia, Mexico, India, Turkey and South Korea. Aitken noted the U.S. forecast is up by a percentage point from what IMS expected as recently as April. "Near-term growth prospects in the U.S. have strengthened in recent months," he said. Aitken cited factors including pharmacies no longer reducing their medication inventory, drugmakers continuing to increase prices 5 percent a year on average despite the recession, and the companies helping poor and uninsured patients continue buying their medicines by expanding assistance programs that cover part of the cost.