John, 33, drinks - anywhere from one beer to a case a day. Sometimes John tries to abstain - but he describes a nervousness or tension in himself, a bleakness that takes over, that often triggers his drinking. John doesn't recognize that he is an alcoholic, or that he has been using alcohol for years as a self-medication to ward off depression.

David, a teenager, has been failing in school for the past several years and now sluffs much of the time. At home he is belligerent and moody and sometimes violently angry.David is withdrawn from his family, spending most of his time in his room listening to rock music and sleeping. His frustrated teachers and parents are now labeling David "lazy"

and "unmotivated," but he is not. David, like John, is depressed.

Both David and John suffer from a chronic, low-grade depression called dysthymic disorder (DD). About 5 million Americans - twice as many women as men - suffer from this relentless illness, which has been with many since childhood. The disorder itself is physical in nature, just like ulcers or high blood pressure.

The symptoms of DD, though they may be mild, are relentless: chronic irritabiity or anger; feelings of sadness and hopelessness; sleeplessness or oversleeping; diminished ability to think or concentrate; redundant negative thoughts; fatigue or decreased energy; loss of interest and pleasure in activities; and, at times, excessive crying and emotionality.

Because these symptoms are not entirely incapacitating, a victim can walk around with the debilitating effects of DD for years without the illness being recognized.

"I had no idea there was something really wrong with me," says one recovered victim of DD. "It's like having walking pneumonia, but worse. At least with pneumonia, you know from the hacking cough and difficulty breathing that there is something physically wrong. But with the depression, I just thought there was something terribly wrong with me. I couldn't control my thinking and will myself to feel good."

A person with DD is usually unaware he has a very treatable illness. Writing about this disorder in American Health (April l988), Winifred Gallagher stresses that many sufferers of DD have repeatedly sought help from physicians and psychotherapists for secondary symptoms such as fatigue or personalilty problems like low self-esteem. But, because of the confusion of physical illness with personality or the stresses of life, many professionals do not make the correct diagnosis or offer the right help.

DD takes a profound toll on its victims and on those people who are closest to them. Marriages often fail without either partner knowing that an insidious depression in one or both persons has hung like a dark cloud over the relationship, confounding the normal difficulty two people have in adjusting to a relationship.

According to researchers, DD victims most likely have a genetic predisposition to this disorder. The depression itself may be set off by external stresses or hormonal changes in the body. These or other unknown factors cause an imbalance in the neurotransmitters or chemicals that govern the electrical transmission of information throughout the brain. This chemical imbalance, in effect, causes messages in the brain to "short-circuit."

Progresssive professionals who treat DD sufferers often suggest a combination of talk and drug therapies.

Antidepressant medication by itself can often relieve mild chronic melancholy within several weeks. One patient summed up the results of drug treatment this way: "I feel like I've been released from a dark hole. My mind has opened up and and I can think again. And, for the first time in years, I can see beyond myself. I'd forgotten there was sunshine and light out there."

Unfortunately, as Gallagher points out, a "longstanding bias against psychiatric drugs can lead doctors and patients to regard medication as a `defeat,' instead of as part of a systematic approach to finding the right treatment." Until recently, she stresses, professionals would not have thought of giving medication to a mildly depressed patient, because the problem was considered a "psy-chological disorder" that needed psychotherapy.

Particularly promising among antidepressants is a new medication called Prosac, which has few side effects and gives many patients a natural "lift." This medication, which has a high success rate in remitting depression, also has appetite suppressant qualities, inhibiting a patient's craving for sugars and carbohydrates.

DD sufferers can also benefit from cognitive therapy, a systematic talk therapy that helps patients correct negative and distorted thoughts that worsen a depression. Although there is some evidence to suggest that prolonged cognitive therapy by itself can sometimes relieve depression, that solution alone is very expensive and often leaves a patient and his family in pain for many months.