Parenterals are sterile solutions or suspensions of drugs administered directly into veins, muscles, or tissues other than the intestine. They have a more rapid onset than other routes due to direct entry into the bloodstream. Parenterals must meet general requirements of isotonicity, sterility, stability, and freedom from particles and toxins. They have advantages of rapid action, avoidance of first-pass metabolism, and use when oral intake is not possible, but also have disadvantages of pain on injection, high costs, and need for trained staff and sterile facilities.