Dexamethasone, Bactrim, and Cefuroxime in Kids: A Dosing Reference

> **Quick Answer:** Dexamethasone for croup: 0.6 mg/kg as a single oral dose (max 16 mg). Bactrim for UTIs: 8 mg/kg/day of the TMP component divided twice daily (max 160 mg TMP/dose). Cefuroxime for ear infections: 30 mg/kg/day divided twice daily (max 500 mg/day). All are weight-based; doses and maxima differ by indication.

Three more pediatric medications worth knowing in detail: dexamethasone, Bactrim (trimethoprim-sulfamethoxazole), and cefuroxime. Each fills a specific clinical niche and has dosing nuances that make a dedicated calculator useful.

Dexamethasone: A More Potent Steroid with Longer Effect

Dexamethasone is a corticosteroid approximately 7 times more potent than prednisolone on a milligram-for-milligram basis. Its longer duration of action — a single dose remains active for 36–72 hours — makes it ideal for conditions where a brief but sustained anti-inflammatory effect is needed.

Croup (Viral Laryngotracheitis)

Dexamethasone has largely replaced prednisolone for pediatric croup because it works as a single dose with no need for repeat administration. Studies show 0.15–0.6 mg/kg as a single oral dose reduces return visits, decreases hospitalizations, and produces faster resolution of stridor and respiratory distress.

Most current guidelines recommend 0.6 mg/kg as the standard dose (max 16 mg) for moderate-to-severe croup. For mild croup, 0.15 mg/kg may be sufficient. The [dexamethasone pediatric dose calculator](/dexamethasone-pediatric-dose-calculator) handles both dose levels.

For a 12 kg child with croup: 0.6 × 12 = 7.2 mg as a single dose. Dexamethasone oral solution is 1 mg/mL; volume = 7.2 mL. The injectable form (4 mg/mL) can also be given orally — same bioavailability via both routes — which is why the calculator allows selection of both formulations.

Asthma Exacerbations

Dexamethasone is increasingly used in place of prednisolone for pediatric asthma exacerbations, particularly for 1–2 day courses. Research suggests 1–2 doses of dexamethasone (0.6 mg/kg/dose, max 16 mg) produce equivalent outcomes to 5-day prednisolone courses, with significantly better adherence.

When using dexamethasone for asthma (rather than croup), the indication affects the dose duration — one or two doses over 24 hours rather than a single dose.

Bactrim (Trimethoprim-Sulfamethoxazole): Coverage for UTIs and MRSA

Bactrim combines trimethoprim (TMP) and sulfamethoxazole (SMX) in a fixed 1:5 ratio. Dosing is based on the TMP component.

Urinary Tract Infections

For uncomplicated lower UTIs in children: 8 mg/kg/day of TMP divided every 12 hours, maximum 160 mg TMP per dose (320 mg/day TMP), for 3–7 days depending on the child's age and infection severity. Longer courses (7–10 days) are used for febrile UTIs with suspected pyelonephritis.

For a 16 kg child: 8 mg/kg/day × 16 kg = 128 mg TMP/day → 64 mg TMP per dose twice daily. Using the standard Bactrim suspension (40 mg TMP/5 mL = 8 mg TMP/mL): 64 ÷ 8 = 8.0 mL per dose.

The [Bactrim pediatric dose calculator](/bactrim-pediatric-dose-calculator) applies TMP-based dosing and converts to mL.

MRSA Skin Infections

Community-acquired MRSA skin infections — abscesses, impetigo, infected wounds — are increasingly common and require a drug with MRSA coverage. Bactrim at 8–12 mg/kg/day of TMP divided every 12 hours covers most community-acquired MRSA strains. Clindamycin is an alternative.

The dose for skin infections uses the higher end: 10–12 mg/kg/day of TMP, with the same maximum.

Important Contraindications and Cautions

Bactrim is **contraindicated in infants under 2 months** (risk of kernicterus from sulfonamide displacement of bilirubin). It's also contraindicated in patients with sulfa allergy (serious cross-reactivity), significant renal or hepatic impairment, and in patients taking methotrexate (Bactrim inhibits folate metabolism, potentiating methotrexate toxicity).

Bactrim has many drug interactions. It can significantly increase potassium (particularly with ACE inhibitors), potentiate warfarin, and cause hypoglycemia in patients on sulfonylureas. These are more relevant in medically complex children.

Cefuroxime: Second-Generation Cephalosporin for Ear and Respiratory Infections

Cefuroxime (Ceftin) is a second-generation cephalosporin with broader coverage than first-generation agents (like cephalexin) but less broad than third-generation agents (like cefdinir). It covers Haemophilus influenzae, Moraxella catarrhalis, and Streptococcus pneumoniae — the main ear infection pathogens — plus many gram-negative organisms.

Ear Infections and Sinusitis

Cefuroxime is used as an alternative to Augmentin for acute otitis media and sinusitis, particularly when a second-generation cephalosporin is preferred over a beta-lactam/beta-lactamase inhibitor combination.

Dosing: 30 mg/kg/day divided every 12 hours, maximum 500 mg/day (250 mg per dose). Course: 10 days for ear infections in children under 6, 5–7 days for older children.

For a 14 kg child: 30 × 14 = 420 mg/day → 210 mg per dose twice daily. Using the 125 mg/5 mL suspension (25 mg/mL): 210 ÷ 25 = 8.4 mL per dose. The [cefuroxime pediatric dose calculator](/cefuroxime-pediatric-dose-calculator) handles this calculation with available suspension concentrations.

Formulation Note

Cefuroxime suspension has a particularly strong, bitter taste that children frequently refuse. Strategies that help: give it cold, follow immediately with a strongly flavored food or drink, don't tell the child to expect it to taste good. Some families mix it with a small amount of strongly flavored food (chocolate syrup, fruity yogurt). If adherence is a consistent problem, discuss alternative agents with the prescriber.

Cross-Reactivity with Penicillin

Cefuroxime, like all cephalosporins, has a theoretical cross-reactivity with penicillin allergy. Modern data suggests the risk is 1–2% for severe penicillin allergy and substantially lower for mild allergies. For children with an uncertain or mild penicillin allergy label, cephalosporins are generally considered safe to use with monitoring.

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Whether you're verifying a dexamethasone croup dose, calculating Bactrim for a UTI, or converting a cefuroxime suspension volume, the calculators on this site apply the relevant guidelines automatically. Use your child's current weight, select the indication that matches the prescription, and match the concentration to the dispensed bottle.