Adam Clarke on Job 20:16

He shall suck the poison of asps - That delicious morsel, that secret, easily-besetting sin, so palatable, and so pleasurable, shall act on the life of his soul, as the poison of asps would do on the life of his body. The poison is called the gall of asps, it being anciently supposed that the poison of serpents consists in their gall, which is thought to be copiously exuded when those animals are enraged; as it has been often seen that their bite is not poisonous when they are not angry. Pliny, in speaking of the various parts of animals, Hist. Nat. lib. xi., c. 37, states, from this circumstance, that in the gall, the poison of serpents consists; ne quis miretur id (fel) venenum esse serpentum. And in lib. xxviii., c. 9, he ranks the gall of horses among the poisons: Damnatur (fel) equinum tantum inter venena. We see, therefore, that the gall was considered to be the source whence the poison of serpents was generated, not only in Arabia, but also in Italy.