What exactly is a halflife??Do people react differently to each pill??
Half life is how long before to only have half of a thing in you. If for example, you take celexa at 20mg, it has a half life of 35h. After 35h 10mg will still be in you (half of 20mg). After another 35h, 5mg still in you (half of 10mg). After another 35h, 2.5mg still in you. It will eventually reaches 0.0...and at this point it will be considered out of you.
Second question: yes unfortunately. So for each med on each person, it's a new try with unpredictable consequences. It sounds awful but there's a list of side effects that 90% of people MAY get and there is nothing really scarry. They will even vanish with time, some often in the first week. And for the ultimate positive effect that you seek, it is also concerned by the unpredictable. Maybe it will help, maybe not. It's like to play the lotto, but you have much more chance to success that to win in any lotto of course. The success chance at each try for the most popular SSRIs is from 60% to 90%