Weight-based drug dosing
Body weight drives pharmacokinetics for most drugs. Volume of distribution, clearance, and protein binding all scale with size, which is why flat fixed doses work poorly at the extremes of body weight.
For most drugs in adults, actual body weight works fine up to around 100 kg. In morbidly obese patients, lipophilic drugs (benzodiazepines, most antibiotics) distribute into adipose tissue and need actual or adjusted body weight. Hydrophilic drugs (aminoglycosides, vancomycin) distribute poorly into fat and should use IBW. When in doubt, check the specific drug's prescribing information.
Common weight-based adult doses
| Drug | Dose | Notes |
|---|---|---|
| Gentamicin (extended interval) | 5-7 mg/kg OD | Adjust for CrCl. Monitor levels. |
| Vancomycin | 15-20 mg/kg Q8-12h | Target AUC 400-600. Use IBW if obese. |
| Enoxaparin (treatment) | 1 mg/kg BD or 1.5 mg/kg OD | Reduce for CrCl <30. Max 100 mg/dose. |
| Heparin bolus | 80 units/kg IV | Follow local protocol for infusion. |
| Alteplase (STEMI) | 0.9 mg/kg (max 90 mg) | 10% as bolus, rest over 60 min. |
Related tools
For IV infusion rate, see IV Drip Rate Calculator. For renal function and dose adjustment, see Creatinine Clearance. For pediatric dosing, see Pediatric Dose Calculator. For ideal body weight, see Ideal Weight Calculator.