Which Body Weight to Use for Drug Dosing

The default is total body weight

Most drugs are dosed on total body weight, including in obesity. A short list of exceptions distribute poorly into adipose tissue and are dosed on ideal body weight, while aminoglycosides in obesity use adjusted body weight. This tool applies the rules to a specific patient and returns the number you would actually use.

The default is total body weight. The exceptions are a short list, and treating IBW as the default is a common and consequential error that underdoses patients.

Ideal body weight

Ideal body weight is a height-derived estimate of lean mass. It was devised for gentamicin dosing and remains the dosing weight for drugs that distribute poorly into adipose tissue. It is also the anchor for adjusted body weight and for the Cockcroft–Gault weight decision.

Ideal body weight comes from the Devine equations, which use only height and sex:

• IBW depends only on height and sex. Two patients of the same height have the same IBW regardless of what they weigh — that is the point of it.
• If total body weight is below IBW, use total body weight. IBW is a ceiling for lean-mass-dosed drugs, not a floor.

Adjusted body weight

Adjusted body weight sits between ideal and total body weight. It reflects the observation that adipose tissue is not pharmacokinetically inert — hydrophilic drugs distribute into roughly 40% of the excess weight above ideal. Adding that fraction back to IBW gives a dosing weight that avoids both the underdosing of pure IBW and the overdosing of pure TBW.

• Adjusted body weight applies to aminoglycosides in obesity. Do not apply it reflexively to every drug in every obese patient — most drugs still use total body weight.
• Vancomycin is the classic counterexample: it is dosed on total body weight even in obesity, not on adjusted body weight.

Where BMI fits in

BMI is a screening index of weight relative to height. In pharmacy it functions mainly as a gate: several dosing-weight rules and drug eligibility criteria — for example whether to use adjusted body weight for an aminoglycoside, or whether a patient qualifies for weight-management pharmacotherapy — are keyed to a BMI threshold. BMI is a screening tool, not a dosing weight. Do not dose a drug on BMI — use TBW, IBW, or AdjBW as the drug requires.

A worked example

A 178 cm male weighs 110 kg. Calculate his ideal and adjusted body weights.

Answer: 178 cm = 70.1 in, which is 10.1 in over 5 feet. IBW = 50 + (2.3 × 10.1) = 73.2 kg. AdjBW = 73.2 + 0.4 × (110 − 73.2) = 87.9 kg.

Since 110 kg exceeds the 73.2 kg ideal body weight, this patient is a candidate for adjusted body weight if the drug being dosed is one where that applies — aminoglycosides in obesity, by the rule below — rather than most drugs, which would still use the full 110 kg.

How to decide, in order

1. Compare total body weight to ideal body weight. If the patient weighs less than IBW, use total body weight regardless of the drug. IBW is never a floor.
2. If total body weight is at or below ideal, the question is settled — use total body weight regardless of the drug.
3. If the patient is above ideal weight, check whether the specific drug is one of the recognised exceptions. Hydrophilic drugs tend toward IBW or AdjBW; lipophilic drugs tend toward TBW. That principle helps predict the rule but does not replace checking the specific agent.
4. Vancomycin uses total body weight even in obesity — it is frequently and wrongly assumed to follow the aminoglycoside rule.
5. For everything else, the default from step one still applies: total body weight.

Common mistakes

• The default is total body weight. The exceptions are a short list, and treating IBW as the default is a common and consequential error that underdoses patients.
• Adjusted body weight applies to aminoglycosides in obesity. Do not apply it reflexively to every drug in every obese patient — most drugs still use total body weight.
• The Devine equations were published for gentamicin dosing and were never validated as a measure of healthy or desirable weight. Do not present IBW to a patient as a weight target.
• Cockcroft–Gault estimates creatinine clearance in mL/min. A laboratory-reported eGFR is a different quantity, uses a different equation, and is normalised to 1.73 m² of body surface area. Do not substitute one for the other when a package insert specifies a CrCl threshold.

Limitations to keep in mind

• Undefined below 152.4 cm (5 feet). PharmCalc blocks the calculation rather than returning an unvalidated extrapolation.
• Binary sex constants with no validated equation for intersex or transgender patients. Document which constant you used and why.
• The correction factor is drug-class specific and empirically derived. Applying the aminoglycoside factor to an unrelated drug has no evidentiary basis.

References

• Devine BJ. Gentamicin therapy. Drug Intell Clin Pharm. 1974;8:650-655.
• Pai MP, Paloucek FP. The origin of the "ideal" body weight equations. Ann Pharmacother. 2000;34(9):1066-1069.
• Bauer LA. Applied Clinical Pharmacokinetics. 3rd ed. McGraw-Hill.
• Bauer LA, et al. Influence of weight on aminoglycoside pharmacokinetics in normal weight and morbidly obese patients. Eur J Clin Pharmacol. 1983;24(5):643-647.
• Pai MP, Bearden DT. Antimicrobial dosing considerations in obese adult patients. Pharmacotherapy. 2007;27(8):1081-1091.
• Rybak MJ, et al. Therapeutic monitoring of vancomycin for serious MRSA infections: a revised consensus guideline. Am J Health Syst Pharm. 2020;77(11):835-864.
• World Health Organization. Obesity: preventing and managing the global epidemic. WHO Technical Report Series 894. Geneva: WHO; 2000.
• WHO Expert Consultation. Appropriate body-mass index for Asian populations and its implications for policy and intervention strategies. Lancet. 2004;363(9403):157-163.
• Nuttall FQ. Body mass index: obesity, BMI, and health — a critical review. Nutr Today. 2015;50(3):117-128.