Jamieson, Fausset and Brown on Hebrews 5:13

13. useth--Greek, "partaketh," that is, taketh as his portion. Even strong men partake of milk, but do not make milk their chief, much less their sole, diet.

the word of righteousness--the Gospel wherein "the righteousness of God is revealed from faith to faith" (Ro 1:17), and which is called "the ministration of righteousness" (2Co 3:9). This includes the doctrine of justification and sanctification: the first principles, as well as the perfection, of the doctrine of Christ: the nature of the offices and person of Christ as the true Melchisedec, that is, "King of righteousness" (compare Mt 3:15).