NEW YORK (CNN/Money) - Billions of tiny pollen spores floating through the air these days mean another spring of sneezing, itching and burning eyes for an estimated 36 million Americans.

But the spring of 2003 brings new twists for allergy sufferers, doctors and insurers, as well as for drug companies fighting for a piece of the $9 billion hay-fever treatment market.

The biggest change involves Claritin, the antihistamine now selling over the counter for about $1 a pill.

The relatively low price has clobbered the drug's maker, Schering-Plough, which sold more than $3 billion worth of prescription Claritin in 2001, when the drug comprised about a third of its sales. Schering -Plough (SGP: Research, Estimates) shares are off 33 percent over the last 52 weeks.

Yet rival prescription drugs such as Zyrtec, from Pfizer, Allegra, made by Aventis (AVE: Research, Estimates), and Schering-Plough's Claritin replacement, Clarinex, have not necessarily benefited.

That's because doctors say that Claritin's availability without a prescription has made health insurers wary of paying for the more expensive treatments.

"Many of the insurance companies have made it difficult to prescribe the other prescription medications," said Dr. Asriani Chiu, assistant professor of allergy at the Medical College of Wisconsin. "Sometimes it just takes a lot of time [to get insurance authorization]."

On the one hand, millions of uninsured people now have access to Claritin because of its availability without a prescription. But people with insurance may actually be paying more -- a two-month supply of Claritin costs about $60 over-the-counter -- than the co-pays for their old prescriptions.

For drugmakers, the full effect of Claritin's new status won't be known until the companies report second-quarter results in July. Aventis last month said first-quarter sales of Allegra rose 2.1 percent from a year earlier. But Zyrtec sales jumped 33 percent to $293 million in the first quarter, Pfizer said.

Figures from NDCHealth, which tracks drug sales, found that overall prescriptions for allergy drugs made by U.S. companies have fallen 30 percent so far this year, partly because of the loss of Claritin. Zyrtec prescriptions fell 3 percent while Clarinex, with the smallest share of the market, rose 13 percent.

"It's tougher this year for the prescription allergy drugs," said Mike Krensavage, a drug analyst at Raymond James & Associates, who said he popped a Claritin Monday morning.

Girish Tyagi, a Thomas Weisel Partners analyst, said this year the market-share war is being won by Pfizer's (PFE: Research, Estimates) Zyrtec, which has 39.5 percent of the prescription allergy drug market. That's followed by Allegra at 31.5 percent and Clarinex, at 16 percent, Tyagi said.

But the beginning and end of allergy season, because of its ties to the weather, differs every year. So early year-over-year comparisons can be misleading.

Still, Tyagi, envisioning a day when this market-share battle is irrelevant, predicts that the Food and Drug Administration will eventually approve all of these drugs for over-the-counter use, spurred by lobbying from health insurers.

"HMOs have an incentive to drive all of these products over the counter," he said.

This season, there's a new allergy drug in town, Merck's popular asthma treatment, Singulair, which has been approved for seasonal allergies. Merck's (MRK: Research, Estimates) pitch: attracting patients who have built up resistance to antihistamines. Singulair, a once-a-day tablet, uses a different compound, rather than a histamine blocker, to ease allergy symptoms.

Merck sold $474 million worth of Singulair in the first quarter, a figure that includes asthma and allergy prescriptions.

Thomas Weisel's Tyagi doubts that Singulair for allergies will be a strong competitor to Zyrtec and Allegra anytime soon. "My overall sense is it's not going to be in the same status," he said.

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Medical College of Wisconsin's Dr. Chiu said that while Singulair can be effective in some patients, she echoed concerns of other doctors who say allergies are often misdiagnosed.

"Fifteen percent of the population has hay fever, and 7 percent have asthma," said Dr. James Li, an allergist and professor of medicine at the Mayo Clinic. "The first step is always to make the correct diagnosis." Some apparent allergy suffers, Li said, simply have a cold.

Over-the-counter Claritin, Li said, will probably lead to an increase in the kind of mistakes that come with self-diagnosis.

Putting a figure on those afflicted with hey fever is not easy. Fifty million Americans suffer from it, according to Merck, which says about half those seek treatment. Dr. Chiu said the figure ranges from 20 million to 40 million, while the American Academy of Allergy, Asthma and Immunology estimates 36 million.

When it occurs, the allergic reaction, an inherited tendency, produces antibodies that release the chemicals that trigger the familiar symptoms of sneezing, itching and burning eyes.

Mold spores are also a common trigger. But allergy sufferers take heart: The cycle of pollination beginning with trees and followed by grasses peaks in the weeks ahead and will slow by late summer.