Nothing in section 201 of this title shall be construed to interfere with any recognized and reputable practitioner of medicine, dentistry, or veterinary surgery in the compounding of his own prescriptions, or to prevent him from supplying to his patients such medicines as he may deem proper, except as hereinafter provided; nor with the exclusively wholesale business of any person, firm, or corporation whose permanent allegiance is due to the United States dealing and licensed as pharmacists, or having in their employ at least one person who is so licensed, except as hereinafter provided; nor with the sale by persons, firms, or corporations whose permanent allegiance is due to the United States other than pharmacists of poisonous substances sold exclusively for use in the arts, or as insecticides, when such substances are sold in unbroken packages bearing labels having plainly printed upon them the name of the contents, the word "Poison", when practicable the name of at least one suitable antidote, and the name and address of the vender.