The last obstacle

It seems to me that the main factor in Portugal's original decision to decriminalize drugs rather than legalize and regulate was the fact that they were party to several international drug conventions.

Don't get me wrong. I think world cooperation is a Good Thing. But it seems to me the focus of these conventions should be on export controls rather than domestic drug policy.

Does anyone know how to amend or pull out from these agreements? I honestly don't know much about this area. Is there a Dummies Guide to Withdrawing from International Drug Conventions?