You are correct, it is not the antihistamines that are used to make meth, so Benadryl is not involved at all. However there is a type of interaction which can make you more restless if you take Benadryl while on meth. This does not hold true for taking Benadryl after meth left the system, then the interaction should not happen. Benadryl does not keep you addicted to any drug. It has no chemical relation to any known street drug. Like you say, it is Sudafed that is involved. If there is a chance taking a milder type of antihistamine say Vistaril, it is often recommended, but oddly in America it is only Benadryl that is OTC, and it is basically a sledgehammer approach to everything where other meds are better. If someone says Benadryl causes drug addiction I would go "Nuhuh" and shake my head.