(FORTUNE Magazine) – You hate the way you feel. A dark cloud hangs over you, blotting out the sunlight. There's no escape, no hope. You're used to handling stress, but this is different; it's out of your control. Ask a friend for help? A friend couldn't possibly know how you're feeling. You should be able to pull yourself up by your bootstraps. You want to snap your fingers and be normal again--but it doesn't work. You feel like a failure. You often think about ending it all.

A little dramatic, perhaps, but so's the truth: Major depressive illness--or the less severe but more persistent form known as dysthymic disorder--affects one in five women and one in ten men during their lifetime. It can occur at any age (and often goes unrecognized in the elderly), and it's common among people who are single or divorced.

Major depression can be triggered by a stressful life event such as a serious illness, or can occur spontaneously with no apparent precipitating factors. For some people it may be a one-time problem. For others it can recur multiple times or even remain present continuously.

Dysthymic disorder, an ongoing condition, can become severe if a major depressive episode occurs at the same time. Major depression can also alternate with manic behavior (bipolar disorder, or the old term, manic-depression). People who have problems primarily in the wintertime may suffer from seasonal affective disorder (SAD).

Signs and symptoms of depression include fatigue or loss of energy, trouble concentrating or making decisions, uncharacteristic irritability, panic attacks, loss of interest in usual activities (including sex), thoughts of suicide, and an increase or decrease in sleep, appetite, or weight.

In my experience, patients, especially type A executives, who have little patience for emotional problems, tend not to seek help immediately when they're experiencing depression. They try to manage it on their own, which rarely works. Depression is not a weakness; it's a medical illness that needs treatment. Although it may be tough to open up to others, obtaining help will deal with the problem faster and certainly more effectively.

The best news about depression is the abundance of good treatments. Medications can help by improving the chemical imbalance of neurotransmitters in the brain. Tricyclic antidepressants and monamine oxidase inhibitors, which have been around for years, do the job, but side effects (weight gain, heart problems, and interactions with other drugs and food) limit their use.

Selective serotonin reuptake inhibitors (SSRIs), introduced in 1987, have revolutionized depression treatment. The list includes Prozac, Paxil, Zoloft, Celexa, and others. (Wellbutrin and Effexor are related.) Newer tetracyclics such as Remeron are useful in certain situations. In all cases it's important to work with a physician who is well versed in these medications and can tailor the drug and its side-effect profile to each patient.

With severe, unrelenting major depression that doesn't respond to medication, electroconvulsive therapy is sometimes used.

St. John's wort, a nonprescription dietary supplement, may improve mild symptoms of depression and mood, but it doesn't have a great effect on major depression and it can interact with certain prescription medications. Some believe Sam-e and 5-HTP may help, but neither has been studied extensively. For people with SAD, regular exposure to ultraviolet light can be a helpful treatment.

If a down period lasts too long or is accompanied by some of the symptoms listed above, talk to a friend or see a doctor. Depression is a serious problem. It's best to consult with your health-care provider rather than try to self-treat or self-medicate--and possibly end up in a more difficult situation.

Donald D. Hensrud, M.D., is director of the Mayo Clinic Executive Health Program. For more on this topic, go to mayoclinic.com. Mayo Clinic offers Executive Health Programs in Jacksonville, Fla.; Rochester, Minn.; and Scottsdale, Ariz.