When Barry Molefsky picked up a prescription at his pharmacy recently, he wondered why there's never a price list posted for prescription drugs so consumers can know the cost before buying, and maybe even comparison-shop.

"Why isn't it done? I cannot think of another product whose price is not displayed at a retail outlet," says Molefsky, a Reston reader, who asked the pharmacist the question. "She said, 'It just isn't done.' "

The skyrocketing cost of prescription drugs has become a burden on consumers, who last year spent more than $203 billion on prescriptions -- up $20.4 billion from the year before, according to the National Association of Chain Drug Stores. Overpricing by pharmaceutical companies and substantial differences in prices for identical drugs at different pharmacies have fueled heated national debate in this presidential election year. But what Molefsky simply wants to know is why drugstores don't advertise their prices on prescriptions.

"It is possible to post prices," says Lindsey Johnson, a consumer advocate at U.S. Public Interest Research Group (U.S. PIRG), citing New York's state law requiring pharmacists to maintain a price list of the 150 top-selling prescription drugs and provide it to customers on demand.

This summer, New York launched a Web site to help consumers shop around for the best prices on prescriptions. It's similar to the one Maryland created in April that compares prices on a monthly basis of the 25 most commonly prescribed medications throughout the state.

But even those efforts fall short of actually price-tagging prescriptions. And Johnson says there are reasons why posting prices is difficult to do -- even if it might benefit consumers.

"It is a very weird market and very confusing," says Johnson, author of "Paying the Price," the U.S. PIRG study released Thursday that surveyed the cost of prescription drugs at nearly 500 pharmacies in 19 states and the District of Columbia. The study found that prescription drug prices vary widely, from store to store, even customer to customer. Uninsured customers, for instance, pay a full retail price that's typically 78 percent more than what the federal government pays for the same medication. The study didn't consider the cheaper prices of generic drugs.

"Consumers don't have information, and different people pay different prices for the same thing at the same store," Johnson says, explaining that prescription prices are often negotiated by insurance companies. Most insured customers, she adds, pay only their $15 to $30 co-payment and never know the price of their medication.

"You don't know if you are getting the best deal," she says, "but even when you are told the price, most people don't go looking for a better price."

Michael Stewart, spokesman for the American Pharmacists Association, says that although price-tagging prescriptions sounds like a good idea conceptually, "it is not like when you drive by gas stations and you're comparing the price of gas. Medications are much more complex."

Since dosages vary in prescriptions, and prices regularly fluctuate, posting a price list would be a logistical nightmare, he says. "The great majority of customers never pay the full price anyway."

But while the APHA doesn't support listing prescription prices, Stewart says it does encourage consumers to call their pharmacists and ask about prices. That provides an opportunity for pharmacists to help customers take advantage of manufacturers' discount programs, Medicaid and other ways to reduce prescription costs, and better enables the pharmacist to monitor the customer's medication profile to avert problems.

"We don't want people to think of buying medications in the same light as shopping for a toaster or toner cartridges for your printer," Stewart says. "The benefits to be derived from actually having to talk to the pharmacist far outweigh the apparent convenience of being able to look at a list of prices on the wall. And it's not that hard to ask the pharmacist, 'How much will it cost?' "

Got questions? A consumer complaint? A helpful tip? E-mail details to oldenburgd@washpost.com or write Don Oldenburg, The Washington Post, 1150 15th St. NW, Washington, D.C. 20071.