the injection is given below the subcutaneous layer of skin, while a 5⁄8-inch needle may be used for people who weigh under 60 kilograms (130 lb). in any case, the skin does not need to be pinched up before injecting when the appropriate length needle is used. history injections into muscular tissue may have taken place as early as the year 500 ad. beginning in the late 1800s, the procedure began to be described in more detail and techniques began to be developed by physicians. in the early days of intramuscular injections, the procedure was performed almost exclusively by physicians. after the introduction of antibiotics in the middle of the 20th century, nurses began preparing equipment for intramuscular injections as part of their delegated duties from physicians, and by 1961 they had "essentially taken over the procedure". until this delegation became virtually universal, there were no uniform procedures or education for nurses in proper administration of intramuscular injections, and complications from improper injection were common. intramuscular injections began to be used for administration of vaccines for diphtheria in 1923, whooping cough in 1926, and tetanus in 1927. by the 1970s, researchers and instructors began forming guidance on injection site and technique to reduce the risk of injection complications and side effects such as pain. also in the early 1970s, botulinum toxin began to be injected into muscles to intentionally paralyze them for therapeutic reasons, and later for cosmetic reasons. until the 2000s, aspiration after inserting the needle was recommended as a safety measure, to ensure the injection was being administered in a muscle and not inadvertently in a vein. however, this is no longer recommended as evidence shows no safety benefit and it lengthens the time taken for injection, which causes more pain. veterinary medicine in animals common sites for intramuscular injection include the quadriceps, the lumbodorsal muscles, and the triceps muscle.