Section 38. In a prosecution under any provision of chapter ninety-four C, for unlawfully manufacturing, dispensing or distributing a controlled substance in violation of any provision of said chapter, it shall be sufficient to allege that the defendant did unlawfully manufacture, dispense or distribute, as the case may be, such alleged substance, without any further allegations, without naming the person to whom it dispensed or distributed, or quantity of the substance; but the defendant shall be entitled to a bill of particulars. In such a prosecution, a defendant relying upon a prescription, written order, receipt, registration, appointment or authority, or exemption as a defense or justification shall prove the same, and until he has proved it the presumption shall be that he is not so justified or authorized.