俱舍論疏 14.217

14.217

論「由此中無至能無染心」，答。夫性罪者，必染心行。為療病時分限飲酒，無染心故，非是性罪。

The treatise answers: "Because herein there is no [...] able to be without a defiled mind." This is the answer. As a rule, a natural offense must be performed with a defiled mind. When drinking alcohol within limits for the sake of curing illness, because there is no defiled mind, it is not a natural offense.