The provisions of section 207 of this title shall not apply to preparations containing not more than two grains of opium or not more than one-quarter grain of morphine, or not more than one-quarter grain of cocaine, or not more than two grains of chloral hydrate in the fluid ounce, or, of a solid preparation, in one avoirdupois ounce, nor shall they apply to preparations sold in good faith for diarrhea and cholera, each bottle or package of which is accompanied by specific directions for use and caution against habitual use, nor to liniments or ointments sold in good faith as such when plainly labeled “for external use only”, nor to powder of ipecac and opium, commonly known as Dover’s powder, when sold in quantities not exceeding twenty grains. The provisions of this section or section 207 of this title shall not be construed to permit the selling, furnishing, giving away, or prescribing for the use of any habitual users of the same any cocaine, salts of cocaine, or preparation containing cocaine or salts of cocaine, or morphine or salts of morphine, or preparations containing morphine or salts of morphine, or any opium or preparation containing opium, or any chloral hydrate or preparation containing chloral hydrate. But the preceding sentence shall not be construed to prevent any recognized or reputable practitioner of medicine whose permanent allegiance is due to the United States from furnishing in good faith for the use of any habitual user of narcotic drugs who is under his professional care such substances as he may deem necessary for their treatment, when such prescriptions are not given or substances furnished for the purpose of evading the provisions of this section. But the provisions of this section or section 207 of this title shall not apply to sales at wholesale between jobbers, manufacturers, and retail druggists, hospitals, and scientific or public institutions.