Intravenous Route

This directly administers the medications to the systemic circulation. It is indicated when a rapid drug effect is desired, a precise serum drug level is needed, or when drugs are unstable or poorly absorbed in the gastrointestinal tract. It is also the route utilized in patients with altered mental status or severe nausea or vomiting, unable to tolerate oral medications.

Advantages:

• Rapid onset of action
• Predictable way of action and almost complete bioavailability
• The problems of oral drug administration can be eliminated by avoiding the gastrointestinal tract
• The best way of administration in very ill and comatose patients, who cannot ingest anything orally

Disadvantages:

• Causes pain
• Chance of infection
• The delivery of protein products that require sustained levels can be difficult.

Intramuscular Route

This can be utilized when oral drug absorption occurs in an erratic or incomplete pattern; the drug has high first-pass metabolism or when the patient is not compliant. A depot preparation of the drug can be given intramuscularly, and the medication dissolves slowly into the circulation to provide a sustained dose over a more extended time. An example includes haloperidol decanoate. Vaccines are also administered via the intramuscular route.

Disadvantages:

• Injection site pain
• The amount of drug administered has to be adjusted according to the mass of the muscle available.
• Peptides get degraded locally.
• Complications – hematoma, abscess, peripheral nerve injury, puncture of a blood vessel leading to inadvertent intravascular administration.

Subcutaneous Route

This is used when the drug’s molecular size is too large to be effectively absorbed in the intestinal tract or when better bioavailability or a faster absorption rate is needed than the oral route. It is easy to administer and requires minimal skills, so patients can often self-administer the medication. Common medications administered subcutaneously include insulin, heparin, and monoclonal antibodies. The rate of absorption of drugs through this route can be enhanced by infiltration with the enzyme hyaluronidase.

The major factors that affect the rate of absorption by this route include size of the molecules (large molecules having slow penetration), viscosity, and the anatomical characteristics of the site of injection (vascularity and amount of fatty tissue).

Disadvantages:

• The rate of absorption is difficult to control.
• Local complications – irritation and pain.
• Injection sites must be changed frequently to prevent the buildup of unabsorbed medication, which could lead to tissue injury.