good drugs :] hehe!
due to synergism
Usually because they are depressed, and the medication is made to take it away. Others take it for recreational purposes, because it can make you feel happy.
MAOI inhibitors (anti-depresents) will almost instantly end an LSD experience. Any hospital or emergency room has access to these. This is usually dicouraged, the best treatment would be to have a friend, who is not tripping, but has experience with the drug, present during the trip in order to help remind the tripper of what is dangerous or safe.
As a general answer, one effective way is to train yourself to stop thinking about the person except perhaps for specific times during the day. Say for 10 minutes at noon and ten minutees at seven in the evening. And during those periods to think of only the pleasant times with him or her and never, ever, ever about the breakup or how bad it is to be without them now. This sounds difficult but you will find yourself improving steadily until it becomes a good habit. Until it becomes a good habit, you can make a little game of it by carrying a little notebook with you, writing the day's date at the top of the page, a little 0 every time a bad thought creeps in that day, and a slash through the 0 when you have pushed the bad thought away. You can push it out of your mind by preparing yourself beforehand by a list of things you enjoy thinking about -- your favorite ice cream, a favorite song, something funny, something beautiful, something nice about yourself, Christmas presents, bunny rabbits, sailboats, a delicious snack, and so on and on. A main principle here is that the mind is something like a computer screen in which you can only have one piece of software appearing there at a time. As the bad thought flashes on the screen, you click your thoughts to something else. And patabum, there you go! Clickety clickin down the lane of life.