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.
Lyrics from the song;
“Truthagen truthagen truthagen makes me happy!
My mom got me Truthagen.
Now I’m awesome!
We have it for lunch and tea every day!