Drug Dose Calculators

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Drug dose is the amount of a pharmaceutical agent administered to achieve a therapeutic effect. Most drugs are dosed by weight (mg/kg), with children typically requiring higher mg/kg doses than adults due to differences in drug metabolism and volume of distribution. Other dosing strategies include per body surface area (mg/m²) for chemotherapy, where BSA = √(height × weight / 3,600) or by Mosteller formula. Loading doses achieve rapid therapeutic concentrations; maintenance doses replace drug eliminated since the last dose. Dose adjustments are required for renal and hepatic impairment when drugs are eliminated by those pathways.

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Weight-Based Dose Calculation

Dose = patient weight (kg) × dose (mg/kg)

Example: amoxicillin 25 mg/kg/day for a 20 kg child: Daily dose = 25 × 20 = 500 mg/day. If divided TID (three times daily): 500/3 = 167 mg per dose (round to available formulation).

Body Surface Area (BSA)

Mosteller formula: BSA (m²) = √(height (cm) × weight (kg) / 3600). Example: 170 cm, 70 kg: BSA = √(170 × 70/3600) = √3.306 = 1.82 m². Common for chemotherapy: e.g., paclitaxel 175 mg/m²; dose = 175 × 1.82 = 318.5 mg.

Loading vs. Maintenance Dose

Loading dose = Vd × target plasma concentration. Vd = volume of distribution. Loading dose achieves rapid therapeutic level. Maintenance dose = Cl × target concentration × dosing interval. Cl = clearance. Interval adjusted to keep concentration within the therapeutic window.

Renal Dose Adjustment

Renally eliminated drugs: dose reduced or interval extended when GFR < 60 mL/min. Cockcroft-Gault equation for CrCl: [(140−age) × weight / (72 × serum Cr)] × 0.85 (if female).

Glossary

Weight-Based Dosing (mg/kg)
Drug dose = weight (kg) × mg/kg; accounts for differences in volume of distribution; used for children and many antibiotics; always check calculated dose against maximum adult dose.
Loading Dose
Large initial dose to rapidly achieve therapeutic concentrations; = Vd × target concentration; needed when waiting 4–5 half-lives for steady state is clinically unacceptable.
Creatinine Clearance (CrCl)
Estimated GFR from Cockcroft-Gault: [(140−age)×weight]/(72×serum Cr) × 0.85 (female); used to adjust doses of renally eliminated drugs; basis for CKD staging.

Frequently Asked Questions

Weight-based dosing: dose = weight (kg) × dose in mg/kg. Example: ibuprofen 10 mg/kg/dose for a 25 kg child: dose = 10 × 25 = 250 mg per dose. Why weight-based: children have different pharmacokinetics than adults — higher total body water percentage (larger volume of distribution for hydrophilic drugs); more active hepatic metabolism per kg body weight; immature renal function in neonates/infants. Weight-based dosing better adjusts for these differences than fixed doses. Upper limit: most pediatric doses have a maximum adult dose — always check that calculated dose doesn't exceed adult dose. Example: if a large adolescent (60 kg) calculates to 600 mg ibuprofen (10 mg/kg), the adult maximum (400–800 mg per dose) applies.

BSA dosing: dose = BSA (m²) × dose per m². Used primarily for: Chemotherapy: most cytotoxic drugs correlate toxicity better with BSA than weight (e.g., carboplatin, cyclophosphamide, paclitaxel, doxorubicin). Some immunosuppressants and biologics. BSA calculation: Mosteller: BSA = √(H × W / 3600). DuBois: 0.007184 × H⁰·⁷²⁵ × W⁰·⁴²⁵. Example: patient 165 cm, 65 kg: BSA = √(165×65/3600) = √2.979 = 1.73 m². Paclitaxel 175 mg/m²: dose = 175 × 1.73 = 302.75 mg. Why BSA: organ volume, cardiac output, and metabolic rate scale more closely with BSA than weight; reduces pharmacokinetic variability.

Loading dose: a large initial dose to rapidly achieve therapeutic drug concentrations. Needed when: target concentration must be reached quickly (serious infection, arrhythmia, seizure). Without loading dose: it takes ~4–5 half-lives to reach steady state. Loading dose = Vd × Cpss_target. Example: digoxin Vd = 7 L/kg, 70 kg patient, target Css = 1.0 ng/mL: loading dose = 7 × 70 × 1.0 ng/mL = 490 mcg ≈ 500 mcg. Maintenance dose: replaces drug eliminated during each dosing interval; keeps concentration in therapeutic window at steady state. MD = Cl × Cpss_target × interval. Relationship: MD (mg) = loading dose × (1 − e^(−kτ)), where τ = dosing interval and k = elimination rate constant.

Renally cleared drugs require dose reduction when kidney function is impaired to prevent drug accumulation and toxicity. Assessment: Cockcroft-Gault equation estimates creatinine clearance (CrCl): [(140−age) × weight / (72 × serum creatinine)] × 0.85 (if female). Example: 70-year-old woman, 60 kg, serum Cr = 1.2 mg/dL: CrCl = [(140−70) × 60 / (72 × 1.2)] × 0.85 = (70 × 60/86.4) × 0.85 = 48.6 × 0.85 = 41.3 mL/min. CKD stages: CrCl 60–90: mild; 30–60: moderate; 15–30: severe; < 15: kidney failure. Drug adjustment: reduce dose proportionally to CrCl ratio (dose_adjusted = normal dose × patient CrCl / normal CrCl); or extend dosing interval. Drugs needing renal adjustment: aminoglycosides, vancomycin, digoxin, metformin (hold if CrCl < 30), renally eliminated antibiotics.