The law of supply and demand is catching up with the emerging retail health clinic business.

With high-profile chains like CVS, Walgreens and Wal-Mart projecting rapid growth for clinics in their stores, supermarkets that hope to keep up are finding it slow going.

While a constant flow of news stories attest to the public's interest in the concept, a combination of factors — including the capitalization of independent clinic operators, long periods before clinics break even and then turn a profit, a shortage of nurse practitioners, varying state laws and regulations, and opposition by physicians — hinder installation and expansion plans.

Last week, the American Medical Association, Chicago, said it would ask state and federal agencies to investigate potential conflicts of interest posed by joint ventures between store-based health clinics and pharmacy chains.

For all of those reasons, “it may not move as fast as everyone originally thought it would, but I think the concept is here to stay,” said Ron Peters, vice president of pharmacy at Minyard Food Stores, Coppell, Texas. “It's going to be widespread.”

CVS/pharmacy, Woonsocket, R.I., is continuing to increase the more than 170 locations it now has, stemming from its purchase last year of MinuteClinic, Minneapolis, and Walgreen Co., Deerfield, Ill., expects to increase its number of clinics from about 50 now to over 400 by the end of next year as a result of its May acquisition of Take Care Health Clinics, Conshohocken, Pa. Wal-Mart Stores, Bentonville, Ark., which now has 75 clinics operated by other companies, said it plans to open 400 within three years, and 2,000 in five to seven years.

Even for those chains, things could be better. At a recent conference, both CVS and Wal-Mart acknowledged that profits of their clinics are lagging, and a CVS/MinuteClinic executive said it couldn't open some clinics because of a lack of nurse practitioners and physician assistants. By acquiring Take Care and funding its expansion, a Walgreens spokesman said that the company would be able to move faster than if the clinic company had remained independent.