When administering or prescribing medications, it is crucial for providers to understand the rationale behind the recommended dosage. Medication dosage must take into account the anticipated effects of the drug on a patient and their unique body. One factor to consider for determining a medication dosage is patient weight.

Wrong-weight errors in medication dosage can be costly for patients and providers. Globally, the cost associated with medication errors has been estimated at $42 billion annually (Donaldson et al., 2017). These errors have been extensively studied in pediatric populations, revealing medication error rates as much as 8x higher when compared to adult populations (Canales-Siguero et al., 2025). Consequently, the use of pediatric dosing charts has been popularized for drugs commonly used in emergent scenarios (Serralabós-Ferré et al., 2024). In 2009, the Pennsylvania Patient Safety Advisory revealed that anticoagulants, followed by aminoglycoside antibiotics, acetaminophen, dobutamine, and dopamine, are most commonly involved in wrong-weight errors (Pennsylvania. Patient Safety et al., 2009). This is a costly trend that also affects antibiotic stewardship. One retrospective cohort study of vancomycin prescribing in the emergency department (ED) showed that only 19.6% of patients received a consensus-guideline recommended dosing regimen (15–20 mg/kg), reflecting an area of weakness in prescriber training and safety checks (Zhou et al., 2016).

Pharmaceutical dosing is determined in phase 1 or early phase 2 clinical trials with the determination of the average effective dose (ED50) and average toxic dose (TD50). These terms represent the lowest drug dosages, calculated in milligram of study drug per kilogram of patient bodyweight, at which 50% of study participants will experience therapeutic benefit (ED50) or toxic effect (TD50), respectively. ED50 and TD50 are then used to determine a window of dosage in which a drug will treat the intended symptoms (>ED50) without causing too many new toxic symptoms (<TD50) (Muller & Milton, 2012). This ratio is known as the therapeutic index (TI) (Muller & Milton, 2012).

This type of testing usually follows the determination of LD50, the dose at which 50% of test subjects experience lethal consequences, which is studied in animal models and estimated for humans. A close evaluation of these numbers allows researchers to determine a safety recommendation and dosing regimen that typically reflects the scheme used in successful phase III studies. Substances can be classified according to toxicity level as determined by established scales, including the Hodge Sterner Scale and the Gosselin, Smith and Hodge Scale (Konan et al., 2022). These scales also consider medication dosage according to the weight of the patient or subject.

Finally, regulatory committees will approve a dosing regimen with additional dosing adjustments accommodating pharmacokinetic (PK) or pharmacodynamic (PD) concerns (Powell et al., 2021). These evaluations frequently take into account the morbidity of the condition the medication is intended to treat, with more severe conditions tolerating more toxic drugs.

Body weight, composition, and renal function all affect the absorption (the ability to move a drug into the blood stream), distribution (the ability of a drug to move into the target tissue), metabolism (the body’s ability to process a drug into its active or inactive form), and elimination (the body’s ability to excrete or remove a drug from its system) of medication, making these factors relevant for medication dosage. Additionally, drug–drug interactions can induce variations in any of these PK/PD factors, potentially making a medication more or less active in the body. In turn, this can lead to unwanted side effects or less effective treatment. If these impact a medication’s safety or effectiveness in initial studies, the medication’s manufacturer may recommend determining dosage based on patient weight. Medications with smaller TIs or severe toxic symptoms are more frequently administered with weight-adjusted dosing.

References

Canales-Siguero, M. D., García-Muñoz, C., Caro-Teller, J. M., Piris-Borregas, S., Martín-Aragón, S., Ferrari-Piquero, J. M., Moral-Pumarega, M. T., & Pallás-Alonso, C. R. (2025). Electronic Prescribing in the Neonatal Intensive Care Unit: Analysis of Prescribing Errors and Risk Factors. Journal of Medical Systems, 49(1). https://doi.org/10.1007/s10916-025-02161-8

Donaldson, L. J., Kelley, E. T., Dhingra-Kumar, N., Kieny, M. P., & Sheikh, A. (2017). Medication Without Harm: WHO’s Third Global Patient Safety Challenge. Lancet, 389(10080), 1680-1681. https://doi.org/10.1016/s0140-6736(17)31047-4

Konan, A., Golly, K., Kra, A., Adima, A., & Lohoues, E. (2022). Phytochemical Screening and Toxicity Assessment of <>Imperata cylindrica (L.) P. Beauv. (Poaceae) Raw Extracts with Brine Shrimp ( Artemia salina ) Lethality Assay. Journal of Biosciences and Medicines, 10, 153-171. https://doi.org/10.4236/jbm.2022.108014

McCallum, L., Lip, S., & Padmanabhan, S. (2014). Chapter 18 – Pharmacodynamic Pharmacogenomics. In S. Padmanabhan (Ed.), Handbook of Pharmacogenomics and Stratified Medicine (pp. 365-383). Academic Press. https://doi.org/https://doi.org/10.1016/B978-0-12-386882-4.00018-9

Muller, P. Y., & Milton, M. N. (2012). The determination and interpretation of the therapeutic index in drug development. Nat Rev Drug Discov, 11(10), 751-761. https://doi.org/10.1038/nrd3801

Pennsylvania. Patient Safety, A., Ecri, & Institute for Safe Medication, P. (2009). Medication errors : significance of accurate patient weights. Pennsylvania Patient Safety Authority.

Powell, J. R., Cook, J., Wang, Y., Peck, R., & Weiner, D. (2021). Drug Dosing Recommendations for All Patients: A Roadmap for Change. Clinical Pharmacology &amp; Therapeutics, 109(1), 65-72. https://doi.org/10.1002/cpt.1923

Serralabós-Ferré, J., Castillo-Gomez, F., Sensarrich-Roset, M., Barceló-Carceller, I., & Gonzalez-Rioja, X. (2024). Use of Standardized Dosing Sheets in Pediatric Emergency Care: Experience in a Third-Level Hospital. Clin Pediatr (Phila), 99228241301842. https://doi.org/10.1177/00099228241301842

Zhou, Q., Pan, S.-D., Zhu, L.-L., Chen, M., & Xia, P. (2016). Weight-based dosing in medication use: what should we know? Patient Preference and Adherence, 549. https://doi.org/10.2147/ppa.s103156