Immunotherapy, as allergy shots and under-the-tongue treatments are called, involves giving controlled doses of the allergen to teach the immune system to tolerate it. That can lead to longer-lasting relief.

Some studies have shown that three years of immunotherapy provides substantial relief for two years after treatment is stopped, in some cases for longer. Some studies also suggest that immunotherapy can reduce the risk of developing asthma.

But only a small percentage of people with respiratory allergies get the shots.

One reason is that people have to visit the doctor for shots at frequencies ranging from twice a week to once a month for several years. Another is that immunotherapy, while now broadly accepted as studies have accumulated, has had its doubters.

“Allergists always had a bad rap,” said Dr. David P. Skoner, chief of allergy, asthma and immunology at Allegheny Health Network in Pittsburgh. “We were called quacks and shot doctors. The reason was the science wasn’t there.”

The allergen extracts used for the shots are approved by the F.D.A. on the basis of their safety, purity and potency. But because immunotherapy has been around for so long, extracts generally do not go through clinical trials to prove they can actually help tame allergies.

Merck and Stallergenes, by contrast, have done controlled clinical trials and are seeking approval on the same basis as any other drug. That could enhance the credibility of the treatments.

In Merck’s biggest trial, involving 1,500 adults and children who are allergic to Timothy grass, those who took the tablets had a 20 percent reduction in symptoms during one allergy season compared with those who received a placebo. They also used antihistamines or other symptom-relieving drugs less often. Stallergenes’ results were in the same ballpark.