Neonates are the most pharmacologically vulnerable patients. Their immature organ function, unique body composition, and rapidly changing physiology create a drug response profile profoundly different from older children and adults. This guide covers the PK basis of neonatal drug dosing, neonatal opioid withdrawal syndrome (NOWS), surfactant therapy, and drugs to avoid or use with extreme caution.
Neonates differ from adults and older children across all four ADME processes. These differences are most pronounced in preterm neonates and in the first week of life, then evolve rapidly over months as organs mature.
Gastric pH near-neutral (acid-labile drugs have higher BA); delayed gastric emptying; immature intestinal enzymes; large body surface area (transcutaneous absorption significant)
Higher total body water (TBW): ~85% (preterm) → 75% (term) → 60% (adult); lower fat and protein binding (albumin and AGP reduced 50–80%); larger Vd for water-soluble drugs
CYP enzymes severely reduced at birth (CYP3A4 ~20% adult; CYP1A2 minimal; CYP2D6 absent until ~2–4 weeks); UGT activity very low (glucuronidation impaired); sulphation is relatively preserved
GFR ~2–4 mL/min/1.73m² at birth (term); ~0.5–1 preterm; matures to adult by 6–12 months; prolonged t½ for all renally eliminated drugs
See paediatric pharmacology guide for age-group comparison and allometric scaling for dose calculation principles.
Renal drug elimination is the most critical PK difference in neonates. At birth, nephron endowment is complete (except in extremely preterm neonates <24 weeks) but glomerular filtration and tubular function are profoundly immature.
| Age | GFR (mL/min/1.73m²) | Gentamicin t½ | Dosing interval |
|---|---|---|---|
| Preterm 26–28 weeks | ~0.5–1 | 18–30 h | 48–72 h |
| Preterm 28–32 weeks | ~1–2 | 12–18 h | 36–48 h |
| Term neonate (day 1–7) | ~2–4 | 8–12 h | 24–36 h |
| Term neonate (1–4 weeks) | ~10–20 | 4–8 h | 18–24 h |
| Infant 1–6 months | ~40–80 | 2–4 h | 12–18 h |
| Adult | ~120 | ~2 h | 8 h (standard) |
TDM is essential for aminoglycosides and vancomycin in all neonates. Consult local neonatal formulary (BNF for Children, Neofax, Lexicomp Neonatal) for weight- and gestational-age-specific dosing. See TDM guide.
CYP450 enzyme expression follows a complex developmental timeline that significantly affects drug metabolism in neonates:
NOWS (formerly NAS — Neonatal Abstinence Syndrome) occurs when a neonate exposed in utero to opioids (or other CNS depressants — benzodiazepines, SSRIs, alcohol) develops withdrawal symptoms after birth and separation from the drug supply.
Signs (FNASS assessment): high-pitched crying, excessive sucking, poor feeding, tremors, increased muscle tone, sneezing/yawning, sweating, tachypnoea, fever, vomiting/diarrhoea, rarely seizures. Onset varies by opioid:
Management — non-pharmacological first: rooming-in with mother (reduces pharmacological treatment need by 50%+), breastfeeding if no contraindication, skin-to-skin contact, reduced stimulation, swaddling. Pharmacological: oral morphine solution (titrated by FNASS score); methadone (longer t½ allows once/twice-daily dosing); clonidine (adjunct — reduces adrenergic symptoms). Phenobarbitone for seizures or non-opioid NAS. See opioid safety guide.
Pulmonary surfactant deficiency is the primary cause of RDS in preterm neonates (<34 weeks). Exogenous surfactant replacement therapy, combined with antenatal corticosteroids and respiratory support, has dramatically reduced RDS mortality.
Preparations available: Poractant alfa (Curosurf) — porcine extract, high phospholipid concentration, most evidence; Beractant (Survanta) — bovine; Calfactant (Infasurf) — bovine bronchoalveolar lavage. All require storage at 2–8°C and warming to 37°C before use.
Administration — LISA/MIST method (preferred in stable neonates): thin catheter introduced into trachea via laryngoscopy without intubation; surfactant instilled while neonate breathes spontaneously on CPAP → minimises lung injury from mechanical ventilation. INSURE method: brief intubation → surfactant → rapid extubation to CPAP.
Antenatal betamethasone: 2 × 12 mg IM doses 24 hours apart administered to mother before preterm birth — induces fetal surfactant synthesis (SP-B, SP-C, phospholipids); reduces RDS severity by ~50%, reduces mortality, reduces IVH. See pregnancy and drugs guide.
Key principles for safe drug dosing in neonates:
| Drug | Risk | Alternative |
|---|---|---|
| Chloramphenicol | Grey baby syndrome (immature UGT) | Avoid; use if essential with TDM |
| Benzyl alcohol (preservative) | Gasping syndrome, metabolic acidosis | Preservative-free preparations only |
| Ceftriaxone | Bilirubin displacement → kernicterus; Ca precipitation in lungs | Cefotaxime |
| Codeine | CYP2D6 absent → no active metabolite (no analgesia); ultra-rapid metabolisers: fatal morphine accumulation | Morphine, paracetamol |
| NSAIDs (non-indomethacin) | Renal vasoconstriction in low-GFR neonates; indomethacin used for PDA closure | Paracetamol for analgesia |
| Fluconazole (<29 weeks) | Limited safety data; use prophylaxis protocols only in high-risk preterm | Nystatin (topical antifungal prophylaxis) |
GFR is 2–4 mL/min/1.73m² at term birth (adult: ~120 mL/min/1.73m²). Preterm neonates have even lower GFR proportional to gestational age (~0.5–1 at 26–28 weeks). Aminoglycoside t½ is 8–30 hours in neonates (vs 2 hours in adults) — extended dosing intervals (24–72 hourly) are standard. TDM is essential. GFR matures to adult values by 6–12 months postnatal age.
NOWS occurs when a neonate exposed in utero to opioids develops withdrawal after birth. Signs: high-pitched cry, tremors, poor feeding, sweating, tachypnoea, seizures. FNASS score guides treatment. First-line: non-pharmacological (rooming-in, breastfeeding, skin contact). Pharmacological: oral morphine solution titrated by FNASS. Methadone MOUD causes later onset (48h–1 week) and more prolonged NOWS than heroin (24–72h).
Surfactant deficiency causes alveolar collapse in preterm neonates (<34 weeks). Exogenous surfactant (poractant alfa, beractant) replaces the deficient natural surfactant, restoring alveolar stability and improving oxygenation. Administered endotracheally (INSURE method or LISA/MIST). Antenatal betamethasone accelerates fetal lung maturation and reduces RDS severity by ~50%.
Chloramphenicol (grey baby syndrome — immature UGT), benzyl alcohol preservatives (gasping syndrome), ceftriaxone (kernicterus risk in jaundiced neonates — use cefotaxime instead), codeine (CYP2D6 absent — no analgesia; risk of fatal morphine toxicity in ultrarapid metaboliser mothers who breastfeed), NSAIDs except indomethacin for PDA. All must use preservative-free IV preparations.