You got a very good answer from hamster. One last thing that they use is observation. Someone who is severely depressed is likely to have a flat affect, move slowly, and be suicidal. Being suicidal automatically puts you in the severe category. In July 2011, I was suicidal and had an initial series of ECT. I started to improve after the 3rd treatment. I have had ECT before and this time we went with maintenance ECT so right now I am having ECT every two weeks. It keeps me pretty stable. Another reason it is not a first line of treatment is that it requires, for many, hospitalization. It can be done outpatient if after some treatments it is evident that they do well afterwards and don't need inpatient observation. But, the treatment requires a psychiatrist trained in ECT, anesthesiologist, nurse, recovery nurse. It is more expensive than most pill cocktails. For me, I go in and am in a pre-op room to get my IDband, intake interview, IV. Then on to the ECT room where electrodes, cardiac monitor, oxygen monitor are applied. Then the anesthesiologist puts me on oxygen and I am put out and a muscle paralytic is given. ECT stimulus of just a few seconds is given and there should be a seizure of about 60 seconds. It shows up on the EEG and your toes wiggle. On to the recovery room for about 30 minutes, back to the intake room for 30 minutes and then on home where I sleep for several hours. About 100,000 people a year are given ECT. It is 80% effective.