Why do obese patients need a special dosing weight?

Drugs distribute differently in fat and lean tissue. A water-soluble drug stays mostly in lean mass, so dosing on total weight overdoses it, while a fat-soluble drug spreads into adipose tissue. Choosing actual, ideal, adjusted, or lean weight matches the dose to where the drug actually goes.

How are the body weights calculated?

Ideal body weight uses the Devine formula from height and sex. Adjusted body weight adds 40 percent of the difference between actual and ideal weight. Lean body weight uses the Janmahasatian equation, which incorporates BMI. Actual body weight is simply the measured weight.

Which drugs use which weight?

As a general pattern, aminoglycosides use adjusted body weight, vancomycin and treatment-dose heparin use actual weight, neuromuscular blockers like rocuronium use ideal weight, and propofol induction uses lean body weight. The tool applies the recommended weight for the class you choose.

What do the obesity classes mean?

BMI bands define them: 25 to 30 is overweight, 30 to 35 is class I obesity, 35 to 40 is class II, and 40 or above is class III, also called severe obesity. The class signals how far actual weight has diverged from ideal and how much the dosing-weight choice matters.

Can I prescribe directly from this?

No. This is a teaching and reference aid using common guidance defaults. Real dosing also depends on renal and hepatic function, the indication, therapeutic drug monitoring, and local protocols. Always confirm against the product information and a clinical pharmacist before prescribing.

Obesity Class & Drug Dosing Weight Selector

In an obese patient, choosing the right weight to dose by is often more important than the dose equation itself. A drug that stays in lean tissue must not be dosed on total body weight, and a drug that spreads into fat must not be dosed on ideal weight. This selector computes every relevant weight and recommends the one your drug class needs.

How it works

The tool first finds BMI and the four body weights. Ideal body weight comes from the Devine formula:

IBW (male)   = 50.0  + 2.3 x (height_in - 60)
IBW (female) = 45.5  + 2.3 x (height_in - 60)

Adjusted body weight corrects ideal weight upward by a fraction of the excess, using the standard correction factor of 0.4:

AdjBW = IBW + 0.4 x (ABW - IBW)

Lean body weight uses the Janmahasatian equation, which folds in BMI so it behaves sensibly at the extremes:

LBW (male)   = (9270 x ABW) / (6680 + 216 x BMI)
LBW (female) = (9270 x ABW) / (8780 + 244 x BMI)

Each drug class is mapped to one of these weights based on how the drug distributes and clears.

Worked example and notes

A 110 kg, 175 cm male has a BMI of about 35.9, placing him in obesity class II. His ideal weight is roughly 70.6 kg and his adjusted weight roughly 86.4 kg. For an aminoglycoside the tool recommends adjusted body weight, so the dose is calculated on about 86 kg rather than the full 110 kg, avoiding a large overdose of a drug confined to lean tissue.

These mappings are general guidance defaults. The correct dosing weight can change with renal function, the specific indication, and therapeutic drug monitoring, and local protocols sometimes differ. Always confirm against the product information and a clinical pharmacist before prescribing.