Synth. Commun. 33(15), 2593–2598 (2003) (https://www.thevespiary.org/rhodium/Rhodium/pdf/h3po2-i2.n-dealkylation.pdf)
(https://www.thevespiary.org/rhodium/Rhodium/pdf/h3po2-i2.n-dealkylation.pdf)10.1081/SCC-120021978 (http://dx.doi.org/10.1081/SCC%2D120021978)
I inferred that amphetamine could perhaps be formed from methamphetamine (which is an N-methylated secondary amine) if either the precursor (pseudo)ephedrine or the product was demethylated.
The -OH reduction of the ephedrine WILL predominate completely, before demethylation occurs.
A mixture of hypophosphorous acid (H3PO2) and iodine in acetic acid can selectively cleave the N-alkyl bond in a variety of substituted heterocylic compounds in good to excellent yields without any damage to amide bond present in the substrates.
Note that the demethylation of substrates are tertiary amines.