From the song; 'Hey Boys and Girls' by Evermore.
This is perhaps best understood as a commentary on the use of socially accepted mind-altering
drugs, the most famous of which are probably Prozac and Ritalin. Note how the lyrics say it "makes me
happy" and "now I'm awesome" which strongly suggest it affects the user's mood.
The literal meaning of "truthagen" would be something that generates
truth, which while nonsensical is generally positive-sounding. This term is probably a further commentary on the nonsensical names of designer
drugs such as the aforementioned Prozac and Ritalin.