Pseudoephedrine's formula is: C10H15NO and methamphetamine's is: C10H15N. The only differ by an oxygen molecule yet have different effects.
The reason I ask is because I have a relative in the States who suffers from narcolepsy. He used to live here in Europe and the doctor couldn't give him anything stronger than pseudoephedrine/ephedrine to treat his daytime fatigue. It worked but wasn't as strong. When he moved to America, a physician there put him on Desoxyn, the brand name for methamphetamine hydrochloride. He anecdotally reported that it was incredibly potent, much more than the pseudo he took before.
How is this possible? Not only is methamphetamine more potent than pseudoephedrine but it induces intense euphoria compared to the latter drug.