There could potentially be different explanations as to why we sometimes respond differently to generic medications compared to how we respond to their name-brand counterparts, but I think that the most likely explanation as to why generic venlafaxine affects you differently than how Pristiq affects you is that venlafaxine is the generic version of Effexor, and not Pristiq. Desvenlafaxine is the generic name for Pristiq, though I'm not sure whether or not a generic version of this medication is commercially available as of yet here in the United States. Desvenlafaxine (Pristiq) is the major active metabolite of venlafaxine (Effexor), and although both medications are similar from a pharmacological standpoint (both medications, for instance, inhibit the reuptake of serotonin and norepinephrine), they're still different chemicals and might have some differences in how they affect you, despite their similarities.