Generic Name

Glycerin

Mechanism

Glycerin (or glycerol) is a trihydric alcohol that functions primarily as a humectant, lubricant, and penetrating agent.
• Water‑binding: Glycerin’s three hydroxyl groups attract water molecules, creating an osmotic gradient that draws fluid into tissues.
• Osmotic laxative effect: In suppositories and enemas, it increases the water content of fecal matter, softening stool and stimulating peristalsis.
• Lubrication: By reducing surface friction, it facilitates rectal or vaginal passage of medication or reduces friction in external genitalia.
• Blood‑plasma stabilizer: In some injectable formulations, glycerin acts as a cryoprotectant, preserving protein conformation by disrupting water‑protein interactions.

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Pharmacokinetics

• Absorption
• *Oral*: Limited systemic absorption (<10 %). Predominantly remains in the GI tract to exert a local hypotonic effect.
• *Rectal*: Absorbed locally; minimal systemic bioavailability.
• *Topical*: Absorbed in small quantities; mainly acts dermally or mucosally.
• Distribution
• Widely distributed in body fluids due to high solubility; plasma concentrations stay below therapeutic dosing.
• Metabolism
• Metabolized by glycerol kinase to di‑ or mono‑glycerides, then to triglycerides or phospholipids for energy utilization.
• Elimination
• Renally excreted unchanged or as metabolites; half‑life ∼4–6 h for IV preparations; oral half‑life ∼1 h (mostly local effect).

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Indications

• Gastrointestinal
• Relief of constipation, especially in infants and patients on opioid therapy.
• Adjunct in the management of fecal impaction via glycerin suppositories or enemas.
• Dermatologic
• Moisturizing agent for xerotic or chapped skin.
• Lubricant in medical procedures (e.g., catheter insertion, vaginal examinations).
• Pharmaceutical
• Preservative and viscosity enhancer in topical creams, ointments, and oral suspensions.
• Cryoprotectant in injectables (e.g., anti‑viral or antibody preparations).

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Contraindications

• Contraindications
• *Rectal* or *topical* use in patients with severe rectal or vaginal infections (e.g., candidiasis) that could be exacerbated by fluid influx.
• Use *off‑label* in newborns <2 weeks old for the first 24 h; avoid as a major excipient.
• Warnings
• Risk of diarrhea: Excessive dosing can lead to loose stools or osmotic imbalances, especially in dehydrated patients.
• Allergic reactions: Rare hypersensitivity reactions (urticaria, angioedema).
• Cryoprobe safety: In injectable formulations, high glycerin concentrations may increase viscosity, affecting infusion rates.

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Dosing

*Adjust doses for pediatric patients (<12 kg): 0.025 g/kg per suppository.*

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Adverse Effects

• Common
• Diarrhea, loose stools
• Local irritation or pruritus (rectal, vaginal, or cutaneous)
• Mild nausea when taken orally
• Serious (rare)
• Hyperglycemia due to increased carbohydrate load (e.g., infants on glycerin‑containing medications).
• Allergic dermatitis or anaphylaxis (very uncommon).
• Hemorrhage if used in patients with coagulopathies and concomitant NSAIDs.

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Monitoring

• Fluid balance: Watch for signs of dehydration or fluid overload.
• Serum electrolytes: Monitor in patients receiving multiple osmotic agents.
• Blood glucose: Especially in diabetic or neonatal populations.
• Vital signs during IV administration: Heart rate, blood pressure to detect hyperosmolar side effects.

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Clinical Pearls

• Use the “first-come‑first‑served” rule: If a patient fails 2 glycerin suppositories within 6 h, consider a 10 % glycerin enema; it can be repeated up to 2–3 times daily.
• Don’t let it interfere with medication absorption: For rectally administered drugs, a glycerin suppository can delay absorption by diluting the drug or displacing it. Time the next dose at least 30 min after glycerin use.
• Avoid over‑hydration in infants: A 10 % solution for enemas can cause electrolyte shifts; limit total fluid volume to <0.5 mL/kg.
• Pregnancy caution: Glycerin is considered Category B; use only if benefits outweigh risks, particularly for topical or suppository routes.
• Pharma advantage: When formulating topical dermatologic products, glycerin’s high viscosity reduces product leakage, improving patient adherence.

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• References
• Goodman & Gilman’s The Pharmacological Basis of Therapeutics (15th ed.)
• UpToDate® “Management of constipation in adults” (2025)
• FDA Guidance for the Use of Excipient Glycerol in Pediatric Medications (2023)