A number of technological advances have been made within the area of parenteral drug delivery, resulting in the event of sophisticated systems that allow drug targeting and therefore the sustained or controlled release of parenteral medicines. Parenteral products are sterile liquids which free from pathogens or solid dosage forms composed of 1 or more active excipient, packaged in single-dose or multidose containers. They are administration by injection, infusion, or implantation into the body. The term parenteral springs from the Greek words “para” meaning beside, and “enterom” meaning intestine, which together indicate something done outside of the intestine but  not by way of the alimentary canal . A drug administered parenteral form, is on injected through the hollow of a fine needle into the body at various sit and to varied depths. The three primary routes of parenteral administration are subcutaneous, intramuscular, and intravenous, although there are others, like intra-cardiac and intra-spinal.

Injections are sterile solutions or suspension of medicine in aqueous or oily vehicle meant for introduction into the body by means of an injectable needle under or through one or more layers of the skin or mucosa. Injection must be sterile, isotonic and free from foreign particles, like dust, fibers etc. they need to be introduced through the same route an equivalent route that they're planned for instance, an oily suspension meant for intramuscular injection could also be very dangerous if it's administered by intravenous injections. Similarly, those strong drugs which are necessary to tend through injection may prove very critical if it's given by intravenous route. The parenteral preparations are free from the contaminating microorganism. Among these sterile dosage forms are the varied small and enormous volume injectable preparations, irrigation fluid intended to wash body wounds or surgical openings, and dialysis solutions. Biologic preparations including vaccines, toxoids and antitoxins, sterility in these preparations is important because they're placed in direct contact with the interior body fluids or tissues, where infection can easily arise. Parenteral dosage forms are those dosage forms which drugs are directly injected into body tissue through one or more layer of skin and mucous. Injections are sterile, pyrogen free preparation intended to be administered parenterally.

There are various routes of administration of parenteral preparations are as follows;

INTRADERMAL INJECTIONS: These are given in between dermis and epidermis. Skin of the left forearm is usually selected for given injection. Gennerally, 0.1 to 0.2 ml of parenteral solution is injected by this route. The route is employed for diagnostic purposes and for testing the sensitivity of the injectables.

SUBCUTANEOUS INJECTIONS: These are made under the skin, into the subcutaneous tissue. The volume of 1.0ml or less is typically injected into the upper arm this is often most vital route, because it is convenient for the patient and therefore the doctor.

INTRAMUSCULAR INJECTIONS: These injections are given into the muscular tissues. The muscles of the soulder, thigh or buttock are usually selected. Mostly, volume upto 2.0 ml is run by this route and will not exceed 4.0 ml at one site. Aqueous or oily suspensions and oily solutions are often administered by this route.

INTRAVENOUS INJECTION:- These injections are given into vein and thus introduced directly into the blood steam. The median basilica vein near the anterior surface of the elbow is typically selected, because it's easily located and connects with the main veins of the arm. The parenteral solution should be isotonic with blood if the volumes of quite 15 ml should be injected. The suspensions and oily injections can't be injected by this route

INTRA-ARTERIAL INJECTIONS: These are almost like intravenous injections and are occasionally used for an instantaneous effect during a peripheral area. These injections are given directly into the artery.

INTRACARDIAC INJECTION: These are given into the guts muscle or ventricle in an emergency only.

INTRATHECAL INJECTION: These injections are made into sub-archnoid spinal anesthesia.

INTRACISTERNAL INJECTIONS: These injections are given in between the primary and second cervical vertebrae. This route is employed to withdraw C.S.F. for diagnostic purposes.

INTRA-ARTICULAR INJECTIONS: These are given into the liquid that lubricate the articulating ends of bones during a joint.

Therefore, parenteral route of administration is that the best route for the delivery of the active pharmaceutical substances with narrow therapeutic index, poor bioavailability especially for those drugs, prescribed to unconscious patients.