Plain insulin route of administration

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Here is a complete, textbook-sourced answer on the routes of administration of plain (regular/soluble) insulin:

Plain (Regular/Soluble) Insulin - Routes of Administration

Plain insulin is the only insulin preparation that can be administered by three routes: subcutaneous (SC), intravenous (IV), and intramuscular (IM). All other insulin types are restricted to SC only.

1. Subcutaneous (SC) - Primary Route

The standard route for outpatient use. Regular insulin is injected 30-45 minutes before a meal to account for its slower onset compared to rapid-acting analogs.

Onset: ~30 minutes
Peak: ~2 hours (50-120 minutes)
Duration: 5-7 hours (for usual doses of 5-15 U)

There is ~25% variability in SC absorption between doses in the same patient - significantly more than rapid-acting analogs (~5%).

"Regular insulin is a short-acting, soluble crystalline zinc insulin whose hypoglycemic effect appears within 30 minutes after subcutaneous injection, peaks at about 2 hours, and lasts for 5-7 hours." - Katzung's Basic and Clinical Pharmacology, 16th Ed.

Important note on U-500 regular insulin: The concentrated 500 units/mL formulation is for SC injection ONLY and must NOT be given IV or IM.

"However, unbuffered, regular insulin (500 units/mL) is for subcutaneous injection only and should not be given by intravenous or intramuscular injection." - Goodman & Gilman's Pharmacological Basis of Therapeutics

2. Intravenous (IV) - The Preferred Route in Emergencies

Regular insulin is the only insulin approved for IV use and is the preferred insulin for:

Diabetic ketoacidosis (DKA)
Hyperkalemia (drives K+ into cells)
Perioperative glucose management
Inpatient hyperglycemia (>180 mg/dL)

Only the unbuffered, 100 units/mL (U-100) formulation can be given IV. It is typically added to normal saline at a concentration of 1 unit/mL for infusions.

"When intravenous insulin is needed (eg, inpatient setting), regular insulin is used; it is particularly useful in the treatment of diabetic ketoacidosis and during perioperative glucose management." - Katzung's Basic and Clinical Pharmacology, 16th Ed.

3. Intramuscular (IM)

Regular insulin can be given IM, but this route is rarely used. It has been shown safe and effective in DKA management in selected cases where IV access is difficult.

Absorption depends on tissue blood flow and can be unpredictable during surgery or in hypoperfusion states
Not FDA-approved as a standard route; used when IV access is unavailable

"In selected cases with good outpatient follow-up, treatment of DKA with intermittent bolus dosing of regular insulin by the subcutaneous or intramuscular route without admission has also been shown to be safe." - Rosen's Emergency Medicine

Summary Table

Route	Regular Insulin	Notes
SC	Yes	Standard outpatient route; U-100 and U-500
IV	Yes	U-100 only; drug of choice for DKA, DKA, hyperkalemia
IM	Yes (U-100 only)	Rarely used; unpredictable in low perfusion states
Inhalation	No	Only for rapid-acting analogs (e.g., Afrezza)
Oral	No	Destroyed by GI proteases

Why Regular Insulin Can Be Given IV (But Not Analogs Routinely)

Regular insulin is a soluble, crystalline zinc insulin in aqueous solution. Rapid-acting analogs (lispro, aspart, glulisine) are also technically suitable for IV use but regular insulin remains the standard choice for IV administration in clinical practice. Long-acting insulins (NPH, glargine, detemir, degludec) must never be given IV due to their modified formulations (precipitates, acidic pH, protamine complexes).

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