1. Drugs can be injected under the skin (subcutaneous), into the muscle (intramuscular) and directly into a vein (intravenous). Injecting directly into a vein produces the quickest reaction that affects the whole body. Subcutaneous injections are used for topical effects such as for local anaesthetics.Three other ways in which drugs can be injected are by mouth (orally), by inhalation and through the anus (rectally)....