Injections in Pharmaceutics: Definition, Classification, Preparation, Administration Routes, and Importance in Drug Delivery System

 

Definition of Injections

In pharmaceutics, injections are sterile preparations intended to be administered into the body through the skin or mucous membranes using a syringe and needle or infusion device. These preparations may be solutions, suspensions, emulsions, or dry powders that require reconstitution before use.

Pharmaceutical Definition:

An injection is a sterile, pyrogen-free preparation intended for administration by parenteral routes, bypassing the alimentary canal, to achieve a rapid or controlled therapeutic effect.

The term “parenteral” means “outside the intestine,” referring to any route that avoids the gastrointestinal tract.

 

Classification of Injections

Injections can be classified in several ways — based on physical form, route of administration, and purpose.

 

A. Based on Physical Form

1. Injection (Solution):
   Sterile solution ready for administration (e.g., insulin injection).
2. Injection (Emulsion):
   Sterile emulsion where the active ingredient is dispersed in oil or water (e.g., propofol injection).
3. Injection (Suspension):
   Sterile suspension of drug particles in a suitable liquid medium (e.g., hydrocortisone acetate injection).
4. Powder for Injection:
   Sterile powder that must be reconstituted with a suitable solvent before administration (e.g., ceftriaxone sodium injection).
5. Concentrate for Injection:
   Requires dilution before administration (e.g., potassium chloride concentrate).

 

B. Based on Route of Administration

1. Intravenous (IV): Into a vein; produces rapid systemic action.
2. Intramuscular (IM): Into muscle tissue; slower, prolonged absorption.
3. Subcutaneous (SC): Under the skin; suitable for self-administration (e.g., insulin).
4. Intradermal (ID): Into the dermis layer; used for sensitivity tests or vaccines.
5. Intra-articular: Into a joint cavity (e.g., corticosteroids).
6. Intrathecal: Into spinal canal (e.g., spinal anesthesia).
7. Epidural: Into the epidural space for pain control.
8. Intraocular / Intravitreal: Into the eye for ophthalmic treatments.
9. Intraperitoneal / Intracardiac: Specialized hospital or laboratory routes.

 

C. Based on Purpose

1. Therapeutic Injections – For treatment (e.g., antibiotics, analgesics).
2. Diagnostic Injections – For tests and imaging (e.g., contrast agents).
3. Nutritional Injections – For supplying nutrients like vitamins or amino acids.
4. Vaccines and Immunizations – To stimulate immune response.

 

Preparation of Injections

The preparation of injections is a critical pharmaceutical process that requires sterility, accuracy, and safety.

A. General Steps in Preparation

1. Selection of Ingredients:
• Active ingredient – the drug substance.
• Vehicles – water for injection, oils, or other sterile media.
• Additives – stabilizers, buffers, antioxidants, or preservatives (where permitted).
2. Solution or Suspension Preparation:
   Dissolving or dispersing the drug in a suitable vehicle under aseptic conditions.
3. Filtration:
   The prepared solution is passed through sterile membrane filters (0.22 µm) to remove particulate matter and microorganisms.
4. Filling and Sealing:
   The sterile solution is filled into ampoules, vials, or prefilled syringes in a cleanroom environment and hermetically sealed.
5. Sterilization:
   Depending on the formulation, sterilization is achieved by:
• Autoclaving (moist heat): 121°C for 15–20 minutes.
• Dry heat sterilization: 160–170°C for oily preparations.
• Filtration sterilization: For thermolabile drugs.
6. Quality Control Testing:
• Sterility Test
• Pyrogen Test (using rabbits or LAL test)
• Particulate Matter Test
• Clarity, pH, and Assay Tests
7. Packaging:
   Filled into sterile glass ampoules, vials, or plastic containers and labeled with batch details, strength, and storage conditions.

 

Routes of Administration of Injections

The route of administration determines the onset, intensity, and duration of drug action.

Route

Site of Administration

Absorption Speed

Examples / Uses

Intravenous (IV)

Into a vein

Immediate

Emergency drugs, antibiotics

Intramuscular (IM)

Deltoid, gluteal, or thigh muscle

Moderate

Vaccines, hormones

Subcutaneous (SC)

Under the skin

Slow, sustained

Insulin, heparin

Intradermal (ID)

Dermis layer

Very slow

Allergy tests, BCG vaccine

Intrathecal / Epidural

Spinal canal or epidural space

Localized action

Anesthesia

Intra-articular

Into joint cavity

Localized

Corticosteroid injections

Importance of Injections in Drug Delivery System

Injections play a vital role in modern pharmaceutics and clinical therapy. Their importance includes:

1. Rapid Onset of Action:
   Direct entry into systemic circulation ensures immediate effect — essential in emergencies (e.g., cardiac arrest).
2. Accurate Dosing:
   Complete bioavailability (100%) as the drug bypasses absorption barriers of the gastrointestinal tract.
3. For Unstable Drugs:
   Suitable for drugs that are degraded or poorly absorbed orally (e.g., peptides, proteins, antibiotics).
4. Controlled and Sustained Release:
   Some depot or implant injections provide long-term drug release, reducing dosing frequency.
5. Targeted Delivery:
   Enables direct delivery to specific sites (e.g., intra-articular, intravitreal).
6. For Unconscious or Non-Compliant Patients:
   Ideal when oral administration is not possible.
7. High Sterility and Purity:
   Parenteral products are sterile, ensuring patient safety during administration.

 

Limitations of Injections

While injections are powerful, they also have some limitations:

• Pain or discomfort during administration
• Risk of infection or tissue injury
• Need for trained personnel
• High manufacturing and storage costs
• Difficult to reverse once administered