If I gave the impression that a Mexican prescription is not required, my apologies for not being clear. You still (probably) need a Mexican script for the following reasons:

1) The 50-pill exception applies only to meds that are "lawfully obtained." For many meds, you'll need a Mexican script to lawfully obtain them. I can't recall ever reading about U.S. Customs asking for the Mexican script to determine whether the meds were "lawfully obtained," but it's best to err on the side of caution.

(As for Tylenol #1 with codeine from Canada, one doesn't need a Canadian script because that medication is OTC there, and hence, it can be "lawfully obtained" with no script.)

2) If the Mexican cops catch you with meds that are controlled in Mexico, and you don't have a Mexican script, you'll be busted and facing six months to a year in a Mexican jail.

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I don't know about Tramadol, but I do know that Soma is not scheduled under US federal law, even though it requires a prescription. (The same applies to penicillin, among many other drugs.) A handful of states have scheduled it, but my understanding is that Customs does not enforce state controlled substance laws. The 50-pill rule applies only to federally scheduled meds. I don't know how non-scheduled meds that nonetheless require a script in the U.S. are handled by U.S. Customs.