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The four main parenteral routes are intramuscular (IM), intravenous (IV), subcutaneous (SC), and intradermal (ID) - each differing in depth, site, needle angle, volume, and onset of action.
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Here is a thorough overview of the four main parenteral injection routes.

Routes of Injection (Parenteral Administration)

Injection routes diagram showing IV, dermal, subcutaneous, and IM at different tissue depths
Fig. 1.3 - Lippincott Illustrated Reviews: Pharmacology. A: Cross-section showing depth of each injection route. B: Plasma concentration curves comparing IV vs. IM midazolam.

Why Use Parenteral Routes?

Parenteral routes bypass the GI tract entirely, which is useful when:
  • A drug is poorly absorbed orally (e.g., heparin)
  • A drug is unstable in the GI environment (e.g., insulin)
  • The patient is unconscious or cannot swallow
  • A rapid onset of action is needed
  • Precise dose control is required
The trade-off: parenteral administration is irreversible, may cause pain, local tissue damage, and carries infection risk.

The Four Major Routes

1. Intravenous (IV)

  • Depth: Directly into a vein (bypasses all tissue layers)
  • Onset: Fastest of all routes - nearly immediate when given as a bolus
  • Absorption: 100% bioavailability - no absorption step required
  • Volume: Can accommodate large volumes
  • Types:
    • IV bolus - full dose delivered almost instantly
    • IV infusion - drug delivered over time, giving lower peak concentrations and longer duration
  • Examples: rocuronium, many antibiotics, emergency medications
  • Risks: Hemolysis, thrombophlebitis, air embolism, infection

2. Intramuscular (IM)

  • Depth: Into muscle tissue (below subcutaneous fat)
  • Needle angle: 90 degrees
  • Onset: Faster than SC, slower than IV
  • Absorption: Via simple diffusion - rate depends on formulation
  • Types of IM preparations:
    • Aqueous solutions - absorbed rapidly
    • Depot preparations - suspended in non-aqueous vehicles (e.g., polyethylene glycol, oil); the vehicle diffuses out, drug precipitates at the site, then dissolves slowly over days to weeks - ideal for long-acting formulations
  • Common sites: Deltoid, vastus lateralis, ventrogluteal, dorsogluteal
  • Max volume: ~3-5 mL in most sites
  • Examples: vaccines, antipsychotics (depot), some antibiotics

3. Subcutaneous (SC)

  • Depth: Into the subcutaneous (hypodermis) fat layer - below dermis, above muscle
  • Needle angle: 45 degrees (can be 90° in obese patients with longer needles)
  • Onset: Slower than IM due to lower vascularity of fat
  • Absorption: Simple diffusion; provides slow, sustained, and constant drug release
  • Advantages over IV: Minimizes risk of hemolysis or thrombosis
  • Limitation: Should NOT be used for irritating drugs - risk of severe pain and necrosis
  • Max volume: ~1-2 mL per site
  • Examples: insulin, heparin, epinephrine auto-injectors

4. Intradermal (ID)

  • Depth: Into the dermis (below epidermis, above subcutaneous tissue)
  • Needle angle: 10-15 degrees (nearly flat to the skin)
  • Onset: Slowest - minimal vascularity in dermis
  • Primary uses:
    • Allergy skin testing (diagnostic determination)
    • Tuberculin (Mantoux) test (TB screening)
    • Desensitization therapy
  • Volume: Very small - typically 0.05-0.1 mL
  • Expected result: A visible wheal (raised bleb) under the skin confirms correct placement

Quick Comparison Table

FeatureIVIMSCID
DepthVeinMuscleFatDermis
Needle angle15-25°90°45-90°10-15°
Onset speedFastestFastModerateSlowest
Bioavailability100%HighHighLow
Max volumeLarge3-5 mL1-2 mL0.1 mL
Typical usesEmergency, large doseVaccines, depot drugsInsulin, heparinAllergy testing, TB test

Speed of Absorption: IV > IM > SC > ID

The chart in the image illustrates this clearly - IV midazolam reaches a plasma concentration of ~150 ng/mL within ~15 minutes, while the same dose given IM peaks at roughly ~50 ng/mL over a longer, flatter curve.
- Lippincott Illustrated Reviews: Pharmacology, p. 25-26
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