How do you know if you should dose a drug on ideal body weight versus actual body weight in obese or overweight patients? Unfortunately, there is not a straightforward answer, either in veterinary medicine or in human medicine. However, more data are available in human medicine.
In humans, lipophilic drugs have an increased volume of distribution (Vd). This can cause both a delay in reaching steady state during prolonged dosing anda disproportionately long half-life. In other words, an obese patient may take longer to reach the therapeutic concentration than a non-obese patient. The obese patient may also require a longer washout period once the drug has been discontinued.
These effects are variable between patients and depend on many other factors, such as whether the drug is distributed by active transport or passive diffusion and the degree of obesity.
In obese humans, cytochrome P450 activity may be either increased (CYP2D6 and CYP2E1) or decreased (CYP3A4 and CYP2C19) compared to the general population. These enzymes are responsible for the majority of drug metabolism. It is possible that this same effect can be found in obese veterinary patients.
The following drugs may be dosed on adjusted, lean, or ideal body weight in humans:
- Antibiotics: macrolides, fluoroquinolones, glycopeptides, beta-lactams, aminoglycosides
- Antifungals: amphotericin B, fluconazole
- Antivirals: acyclovir
- Digoxin
- Levothyroxine
- Methylprednisolone (consider dosing less frequently)
- Neuromuscular blockers: atracurium, rocuronium
- Remifentanil
- Low molecular weight heparins
This article provides an overview of when human medical doctors use adjusted or ideal body weight to calculate dosage. Because data are lacking for veterinary populations, veterinarians should use their judgment on a case-by-case basis to determine whether to adjust dosing for obese veterinary patients.
References
- Barras M, Legg A. Drug dosing in obese adults. Aust Prescr. 2017 Oct;40(5):189-193. doi: 10.18773/austprescr.2017.053. Epub 2017 Oct 3. PMID: 29109603; PMCID: PMC5662437.
- Bruno CD, Harmatz JS, Duan SX, Zhang Q, Chow CR, Greenblatt DJ. Effect of lipophilicity on drug distribution and elimination: Influence of obesity. Br J Clin Pharmacol. 2021; 87:
- 3197–3205. https://doi.org/10.1111/bcp.14735.
- Gouju, Julien, and Samuel Legeay. “Pharmacokinetics of obese adults: not only an increase in weight.” Biomedicine &
- Pharmacotherapy 166 (2023): 115281. https:// doi.org/10.1016/j.biopha.2023.115281.
- Kane SP. Drug Dosing in Obesity Reference Table. ClinCalc: https://clincalc.com/Kinetics/ ObesityDosing.aspx. Updated July 15, 2021.
By Lauralei Fisher-Cronkhite, PharmD, DICVP, FSVHP
