Phenylephrine, a short-acting vasoconstrictor, is the active ingredient in Neo-Synephrine, a medicine cabinet staple since it entered the market in 1940. A longer-acting compound -- oxymetazoline and xylometazoline -- appeared in the 1960's and is responsible for Afrin's advertised 12-hour relief. (There is also a formulation of Neo-Synephrine containing oxymetazoline.)

"Afrin is safe and effective when used for three days," said a spokeswoman for its maker, Schering-Plough. "We do not support extended use of this product."

Though it is not entirely clear why, the blood vessels in the nasal lining quickly become tolerant to the drugs' shrinking effects. With months of overuse, the sprays choke off blood flow to the nasal membranes and damage them. In some patients with severe cases, Dr. Bhattaharyya said, "the inside of their nose looks like a chemical burn."

Dr. Goldstein said he had seen patients with holes in the nasal septum -- the structure that separates the two breathing passages -- from abuse of the decongestants.

Decongestants do not solve the problem that prompts their use, except in the case of a transient cold. The drugs should not be used for chronic conditions like seasonal or persistent allergies, for breathing obstruction caused by a deviated septum or for a common syndrome called vasomotor rhinitis, an innate hypersensitivity to irritants like chemicals, pollutants or cold air.

These afflictions are better treated with nasal steroids, like Rhinocort or Flonase, which build up their action over time to control chronic stuffiness without the risk of rebound or significant side effects.

For more acute problems, oral decongestants like Sudafed that work over a period of hours are a good choice because they lack the potential for rebound congestion. External strips that hold the nostrils open can also help at night.