Dear Dr. Donohue — On my own, I went to a sleep clinic to be assessed for sleep apnea. After two nights there, I was told I need a CPAP machine. They decided I needed a full face mask because I am a mouth-breather. After the first night, I knew I could not wear that mask. I got to the point where I did not want to go to bed. Finally, after five months of stress, I went to my heart doctor and a lung doctor. They told me not to put myself through such stress. — N.M.

Answer —"CPAP" stands for "continuous positive airway pressure," a device that imparts pressure on incoming air so it can bypass throat obstruction and reach the lungs."Apnea" is a Greek word meaning"no breathing." With sleep apnea, people have periods of 10 seconds or longer when they don't breathe. Often, these people are loud snorers, and the apnea periods are preceded by ever-increasing loud snoring until there's a sudden silence. The silence is the period of no breathing. The apneic period ends with a grunt from the snorer, after which breathing and snoring resume. Lax tissue in the throat is the obstruction to airflow for these people. It's also responsible for snoring.

Sleep apnea leads to daytime sleepiness, a rise in blood pressure and possibly an increased risk of heart disease and stroke.

Weight loss, if applicable, is one way to treat apnea. Your dentist can fashion a device that brings the jaw and tongue forward to relieve throat obstruction. It's called a mandibular repositioning splint. And people often benefit from masks that are not as cumbersome as a full face mask, even if they are mouth-breathers. The choices for masks are many. One type comfortably delivers pressurized air into the nostrils. That's only one example of the many kinds of delivery systems.

Dear Dr. Donohue — Is a vitamin B-12 tablet under the tongue as good as a B-12 shot? — B.D.

Answer — Why are you taking B-12? Do you have pernicious anemia? It's due to B-12 deficiency. With pernicious anemia, a person doesn't make intrinsic factor, a substance that promotes absorption of this vitamin into the blood. A B-12 shot bypasses the need for intrinsic factor. The shot gets the vitamin into the body and into the blood.

In Europe it's common practice to give oral B-12 and under-the-tongue B-12 in very large doses to people who don't make intrinsic factor. Enough of the vitamin from such large doses remedies a B-12 deficiency.

If you don't lack intrinsic factor, you can take the vitamin in many ways: by mouth, under the tongue or by shot. Talk with your doctor to see if there is a compelling reason why you have to take the shot.

Write Dr. Donohue at P.O. Box 536475, Orlando, Fla. 32853-6475.