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Weight-based dosing in obesity is complex and drug-specific. This calculator provides general guidance. Always consult drug-specific literature and consider clinical pharmacist consultation for high-risk medications.
Calculate appropriate dosing weight based on BMI and drug characteristics
Patient's measured weight. Used for most patients with normal BMI. May lead to overdosing in obese patients.
Estimated weight for given height. Devine formula:
Males: 50 kg + 2.3 kg × (height in inches − 60)
Females: 45.5 kg + 2.3 kg × (height in inches − 60)
Accounts for some drug distribution into adipose tissue:
AjBW = IBW + 0.4 × (Actual Weight − IBW)
The 0.4 factor assumes 40% of excess weight contributes to drug distribution.
Fat-free mass. Boer formula:
Males: 0.407 × wt(kg) + 0.267 × ht(cm) − 19.2
Females: 0.252 × wt(kg) + 0.473 × ht(cm) − 48.3
| Drug | Recommended Weight | Notes |
|---|---|---|
| Vancomycin | Actual Body Weight | Use ABW even in obesity; adjust by levels |
| Aminoglycosides | Adjusted Body Weight | Use AjBW if >20% over IBW |
| Heparin (DVT prophylaxis) | Actual Body Weight | Consider higher doses in obesity |
| Enoxaparin (treatment) | Actual Body Weight | Cap at 150 kg; check anti-Xa levels |
| Propofol | Lean Body Mass | Highly lipophilic but rapid redistribution |
| Digoxin | Ideal Body Weight | Does not distribute to adipose |
| Neuromuscular Blockers | Ideal Body Weight | Hydrophilic; actual weight causes prolonged paralysis |
Obesity changes body composition (more fat, relatively less water), affects drug distribution, can alter liver/kidney function, and changes protein binding. Lipophilic drugs distribute into fat; hydrophilic drugs don't, making standard weight-based dosing potentially dangerous.
Use ABW for moderately lipophilic drugs in obese patients (typically BMI >30). It accounts for partial drug distribution into excess adipose tissue. The 0.4 correction factor is commonly used but may vary by drug.
For very short patients, Devine formula can yield negative values. In these cases, use actual body weight or consult alternative IBW formulas. The calculator automatically prevents negative IBW values.
No. While lipophilic drugs distribute more into fat, this doesn't always translate to needing higher doses. Drug clearance, protein binding, and volume of distribution at steady state all matter. Consult drug-specific literature.
Often yes. For drugs with narrow therapeutic windows, consider maximum dose caps based on drug-specific data. Monitor closely with drug levels when available. Consultation with clinical pharmacy is recommended.
Bariatric surgery significantly alters drug absorption (especially oral medications) and may change metabolism. Rapid weight loss also changes dosing needs. These patients require specialized consideration beyond standard BMI-based adjustments.