NEW YORK (CNNMoney.com) - Allergy season is in full swing and, in the $25 billion market for respiratory drugs, GlaxoSmithKline is likely to hold its throne as the asthma king for this season and years to come, despite growing competition.

GlaxoSmithKline's (unchanged at $57.00, Research) top earner, its Advair/Serevent franchise of asthma inhalers, will probably remain the leader in the large respiratory market, analysts say. The franchise does face threats from similar drugs, like Merck's (down $0.22 to $34.29, Research) Singulair and Astrazeneca's (unchanged at $53.89, Research) Symbicort, but the GlaxoSmithKline drugs are expected to hold their own and keep growing.

Sales for the Advair/Serevent franchise rose 12 percent in first quarter, to $1.5 billion, following a 22 percent rise in 2005, to $5.6 billion. Advair/Serevent sales are expected to total nearly $6.9 billion in 2006, according to projections from Bernstein, growing to $9.1 billion in 2010.

Advair/Serevent sales are "storming away," according to another analyst, who did not wish to be identified: "There are always threats around, but I think the franchise as a whole is in very good shape. You can't ask for much more than double-digit growth rates from a drug that's already [close to] $6 billion a year."

The respiratory market includes treatments for asthma, allergies and chronic obstructive pulmonary disease, which is a gradual breakdown of lung function. Datamonitor projects that this market will be relatively flat through 2015, increasing by only about $2 billion overall to about $27 billion.

But sales for both Advair/Serevent and Singulair are expected to keep growing, despite the impending U.S. entry of Symbicort, a respiratory blockbuster in Europe that's made by Astrazeneca. Singulair, an antihistamine that can also be used for asthma, had sales of $801 million in the first quarter, up 8 percent, and $3 billion in revenues for 2005. Ryan, of Deutsche Bank North America, projects that Singulair sales will grow to $3.3 billion in 2006, and to $3.9 billion in 2009.

"Singulair competes in the same market as Advair, but they've coexisted for a long time and I don't think that's going to change," said Barbara Ryan, analyst for Deutsche Bank North America. Ryan said the two drugs function with different chemical mechanisms and treat separate ailments.

Symbicort, an asthma inhaler, has not yet been approved by the Food and Drug Administration. Bernstein analysts project Symbicort sales to total $1.2 billion in 2006 in the European market. But if Symbicort is approved by the FDA and enters the U.S. market, Bernstein projects its sales will rise to $1.3 billion in 2007, increasing to $2.6 billion in 2010, including $1 billion in U.S. sales.

But one of the Symbicort's strongest assets, its adjustable dosage, won't be featured in the U.S. model.

"In Europe, Symbicort managed to take some market share because it has an adjustable dosage, but variable dose treatment hasn't even been applied to the FDA," said Les Funtleyder, analyst for Miller Tabak.

Another respiratory blockbuster is Pfizer's (unchanged at $24.50, Research) Zyrtec, an antihistamine which saw sales grow 23 percent to $421 million in the first quarter of 2006, and had $1.4 billion in revenue for 2005. But Zyrtec isn't seen as a big threat, since it loses patent protection in 2007.

In addition, Schering-Plough (unchanged at $19.08, Research) has its Claritin franchise, but those sales have dropped dramatically since the original drug's patent expired in 2002. Schering-Plough has two other respiratory drugs -- asthma inhaler Foradil, which it markets in the U.S. for Swiss drugmaker Novartis (unchanged at $57.28, Research), and the inhaler Asmanex, which entered the market in late 2005 -- but would not provide sales for either drug. Meanwhile, Sanofi-Aventis' (unchanged at $47.77, Research) Allegra franchise has also lost its patent, and has seen sales decline.

______________________________________________________________________

To see how Schering-Plough continues to make money from its Claritin franchise, click here.