Routes of Drug Administration in Pharmacology

Routes of drug administration include:

• Enteral routes
  • Oral (PO)
  • Sublingual
  • Buccal
• Parenteral routes
  • Intravenous (IV)
  • Intramuscular (IM)
  • Subcutaneous (SC)
  • Intradermal (ID)
• Other routes
  • Topical
  • Inhalation
  • Intranasal
  • Rectal
  • Intrathecal / Intraventricular
  • Transdermal

Oral routes (PO)

• Oral administration involves the most complicated absorption process because it goes through many different absorbing surfaces
• Drugs go through the low ph of stomach (1.5) that inactivates a lot of drugs
• To solve this problem: wide range of oral preparations are available including:
  • Immediate release preparations (which have enteric coating)
  • Extended release preparations: those have many layers and dissolve slowly leading to slower absorption and prolonged duration of action, better compliance and less toxicity compared to immediate release, e.g. morphine half life 2-4 hours 6 times daily, 2 times with extended release)

Advantages

• Easily self administered
• Toxicities and overdose maybe overcome with antidotes like activated charcoal
• Economical

Disadvantages

• Limited absorption to some drugs
• Food affect absorption
• Patient compliance
• First pass metabolism

Example: amoxicillin tablet

Sublingual / Buccal routes

Sublingual/ buccal: placement under tongue or between cheek and gum, drug absorbed to circulation.

Advantages:

• Rapid absorption
• Bypass of harsh GI environment
• Avoidance of first pass metabolism

Disadvantages:

• Limited to certain types of drugs that doesn’t cause irritation
• Limited to drugs that is taken in small doses

Example: sublingual nitroglycerin

Parenteral

• Parenteral introduces drugs directly into systemic circulation through injecting drugs directly into:
  • Veins (IV)
  • Subcutaneous (SC)
  • Muscle (IM)
  • Dermis (ID)
• Used for:
  • Poorly absorbed Drugs from GI like heparin
  • Unstable in GI like insulin
  • Patients unable to take oral like unconscious
  • In circumstances required rapid onset (acutely ill patients)

Advantages: include better control over dose

Disadvantages:

• Irreversibility
• Pain
• Fear
• Local damage
• Infections

Intravenous route (IV)

• Most common parenteral route, Drug goes directly into the bloodstream

Advantages:

• Rapid effect and maximum control over the dose
• Ideal for large volumes

Disadvantages:

• Unsuitable for oily substances
• Bolus injection may result in adverse effects (e.g. acute heart failure)
• Most substances must be slowly injections (hypersensitivity)
• Strict aseptic techniques
• Risk of thrombosis and hemolysis

Examples: vancomycin IV, heparin IV

Intramuscular route (IM)

• Parenteral
• Drug injected in the muscle, and then diffuse into systemic circulation
• Drugs administered IM can be in:
  • Aqueous solutions which absorbed rapidly
  • Specialized depot preps which absorbed slowly providing sustained dose over an extended interval

Advantages: suitable for oily drugs, preferable if patient must self administer

Disadvantages: can be painful, blood creatine kinase increased

Example: benzathine penicillin

Subcutaneous route (SC)

• Parenteral
• Drug injected into subcutaneous fat and diffuse into blood after by simple diffusion, slower than IV route

Advantages:

• Minimize risks of hemolysis or thrombosis associated with IV injection
• Provide constant, slow and sustained effects

Disadvantages: if drug causes tissue irritation, this leads to severe pain and necrosis may occur, so this route not used with drugs causes tissue irritation

Example:

• Insulin (SC)
• Heparin (SC)

Intradermal route (ID)

• Parenteral
• Injection into dermis (under epidermis), have the longest absorption time of the parenteral routes

Used for:

• Allergy for a specific drug
• Diagnostic testing such as tuberculin test to check for TB

Oral inhalational route

• Provide rapid delivery of the drug to lungs respiratory epithelium
• Suitable for drugs that are gases (example: some anesthetics like isoflurane) and those that can be dispersed in an aerosol

Used for:

• Respiratory disorders like asthma, copd
• Anesthesia

Advantages:

• The drug action is as rapid as IV route
• Minimal systemic effects

Disadvantages:

• Most addictive route (drug can go to brain quickly)
• Difficult to use inhalers

Examples: albuterol inhaler

Intranasal route

• Topical administration of drugs directly into the nose
• The drug put into and cross nasal mucosa and it either treat nasal mucosa or go to blood supply and act systemically
• Used to treat nose disorders like allergic rhinitis

Intrathecal / Intraventricular route

• Drug is given directly to CSF fluid using these methods
• Intrathecal: drug injected into CSF through spinal cord
• Intraventricular: drug injected into CSF through brain ventricles
• Brain and spinal cord are protected by BBB which prevent most drugs from going to brain, so intrathecal route is very good in giving drugs to work on brain and spinal cord directly

Used:

• When local and rapid effects of drugs are needed such as if patient has meningitis or encephalitis
• Used if the drug can’t cross the BBB such as high molecular weight antibiotics or anticancer chemotherapy

Intraarticular route

• Drug injected Into the joint space
• E.g. steroid injections given in arthritis to relive joint pain and inflammation

Topical route

• Applied to skin or mucosa
• The action intended to work locally at the skin, eyes, intravaginal

Disadvantages:

• Some systemic absorption can occur
• Unsuitable for drugs with high molecular weight or poor lipid solubility

Example:

• Clotrimazole cream
• Hydrocortisone cream

Transdermal

• Differentiate between intradermal, transdermal
• Applied to skin, crosses skin to systemic circulation
• The intention for the drug to cross the skin and enter systemic blood supply to treat disorder unrelated to the skin

Advantages:

• Bypass first pass effect
• Convenient and painless
• Ideal for drugs that are quickly eliminated

Disadvantages:

• Some pt. allergic to patches
• Limited drugs that can be taken in small daily doses

Examples: nitroglycerin patch, nicotine patch

Rectal (PR)

• Absorbed into rectal mucosa which 50% of rectal region drainage bypasses portal circulation (where first pass metabolism occur) so it is minimized
• Prevent destruction of drug in the GI

Useful if:

• Patient is vomiting
• Drug can’t be taken orally
• Patient unconscious

Disadvantages: drugs may irritate rectal mucosa

Example: diazepam PR