Thujone is one of the "active" ingredients. Of course the high alcohol content is the main one, but many versions use spices that have their own "special" properites. Thujone will not show up in any common drug tests, such as those used for employment. It does have an effect similar to marijuana, but is "smoother". The main source for thujone in Absinthe is wormwood. This is easy to obtain. It has a VERY bitter flavor, hench the use of additional spices such as anise.
The traditional way to drink it is to pour it into a glass, place a slotted spoon over the glass, place sugar cubes on the spoon, and pour water over the sugar. This causes the green absinthe to turn white, or louche. This is the method used during the height of absinthe consumption. The modern Czech method is to pour the absinthe over the sugar cubes into the glass, then light the sugar, and stir it into the absinthe. This does not cause it to louche.
Absinthe is easy to make yourself, but the louche is hard to acheive in homemade absinthe. The stories about blindness and other ill effects are mainly due to the addition of all kinds of harmful adulterants back in the days when its popularity was at its peak. The absinthe available today is of varying quality, most poor. Alot of what is called absinthe actually has no thujone at all.
I've tried several brands of varying quality, strengths, and origins, and have made it on numerous occasions. Not really anything to be afraid of. Again, the alcohol is the thing you need to worry about the most.
This is all from memory, there is plently of info on the net about it. The only place I would trust to buy online, for several reasons, quality the main one, is
http://www.czechabsinth.com/m/distillery.html