depression
"I have been taking Prozac now for 20 plus years.  Originally was told by MD that it would be temporary but my experience has been after trying to be off it for several months, that I need to go back on it.  I put myself under depression but I had anxiety, too, after the birth of my first child.  I developed a real neurosis about sleep, i.e., I couldn't shut my brain off at night and had insomnia, became afraid I'd never sleep again.  Being on the lowest dose of Prozac helped me I would say, completely.  At three weeks I despaired of ever feeling normal again and by the fourth week saw a real improvement, by six weeks was very happy and relieved.
It may be losing its effectiveness somewhat after all these years."