21 C.F.R. § 1306.23
The partial filling of a prescription for a controlled substance listed in Schedule III, IV, or V is permissible, provided that:
[36 FR 18733, Sept. 21, 1971. Redesignated at 38 FR 26609, Sept. 24, 1973, and amended at 51 FR 5320, Feb. 13, 1986; 62 FR 13965, Mar. 24, 1997]