I believe that an adamantine weapon is a "non-magical weapon made from material other than silver," as well as a weapon lacking a "+1 or better magical bonus," so it doesn't get through a werewolf's damage reduction, and by extension, anyone else's.Monster Manual, page 9:
For example, the werewolf (damage reduction 15/silver) takes normal damage from weapons with +1 or better magical bonuses, but not from non-magical weapons made from material other than silver.
FANGO said:Also, adamantine weapons do cut through DR, they have an enchancement bonus and an enhancement bonus is required to cut through DR, therefore they cut through DR. End of discussion. (I'm not going to get into that argument again, because I'm right, and there's nothing in the core rules that suggests differently)
Indeed it does.Does adamantine have an enhancement bonus?
Not exactly. Per the Monster Manual, the Monster Manual II, and the sage, it needs to be a magical enhancement bonus.Is DR bypassed by weapons with enhancement bonuses equal to or better than the number after the slash?
I think not.Therefore, adamantine bypasses DR.
It doesn't have to be in the PHB glossary. A magical bonus is a bonus that is magical. There is a thing called common sense.A poorly written example that doesn't use actual rule-based terms ("magical bonus" isn't in the PHB glossary, while "enhancement bonus" is)
The actual rules state that DR is ignored by weapons with an enhancement bonus, with the caveat that it must be a magical weapon....is hardly something that can trump the actual rules (which state that DR is bypassed by adamantine).

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.