A PC member recently explained (I think in the Psych Meds forum) that a pill needs to contain between 4/5 and 5/4th of the dosage it's said to be. So it's possible that your 100 mg pill from brand A contains 80 mg and the "same" pill from brand B 125 mg. So you can imagine that the effects can be different, because the dosage is different! 125 mg is over 1,5 times 80 mg.

I'll try to find the original post.

Edit: the orginal post referenced this article: http://www.doctoroz.com/article/what...-generic-drugs