How the dose is calculated
Linezolid clearance differs by age, so children under 12 need a higher per-kg total daily dose divided three times a day, while those 12 and older (and adults) use a lower per-kg dose divided twice daily. Oral and IV doses are considered equivalent because of linezolid's near-complete oral bioavailability.
Why does the per-kg dose go down as age goes up? It comes down to drug clearance, not dose "tapering" in the usual sense. Young children eliminate linezolid from their bodies considerably faster (higher weight-normalised clearance) than adolescents and adults — pharmacokinetic studies found that dosing children under 12 the same way as adults (10 mg/kg every 12 hours) left drug levels too low for reliable efficacy. Increasing both the per-kg amount and the frequency (10 mg/kg every 8 hours = 30 mg/kg/day) restores equivalent drug exposure (AUC) to what a 10 mg/kg-every-12-hours adult dose achieves. So the two regimens aren't really "more" and "less" — they're two different mg/kg/day numbers engineered to land on the same effective drug exposure, given how differently the two age groups clear the drug. This is exactly the same logic used for the fixed 600 mg twice-daily adult dose: a 60 kg adult is receiving the equivalent of 20 mg/kg/day, split every 12 hours, same as this calculator's ≥12-years option.
Typical dosing by indication
| Indication | Dose | Frequency | Max |
|---|---|---|---|
| Under 12 years | 30 mg/kg/day | every 8h (3×/day) | 1800 mg/day |
| ≥12 years/adult | 20 mg/kg/day | every 12h (2×/day) | 1200 mg/day |
Linezolid is generally reserved for confirmed or strongly suspected resistant Gram-positive infections (MRSA, VRE) or as an alternative when first-line agents cannot be used — it is not a routine first-choice antibiotic.
Clinical use
- Linezolid covers resistant Gram-positive organisms including MRSA and vancomycin-resistant Enterococcus (VRE), making it valuable when first-line agents aren't options.
- Near-complete oral bioavailability allows straightforward IV-to-oral switching at the same dose once a patient is clinically improving.
- Prolonged use (typically beyond 2 weeks) carries a risk of reversible myelosuppression (thrombocytopenia, anaemia) — full blood counts should be monitored during extended courses.
- A weak monoamine oxidase inhibitor — carries a risk of serotonin syndrome when combined with SSRIs, SNRIs, or other serotonergic drugs; review the medication list carefully before prescribing.
Frequently asked questions
What is linezolid used for?
Linezolid treats serious infections caused by resistant Gram-positive bacteria, including MRSA (methicillin-resistant Staphylococcus aureus) and VRE (vancomycin-resistant Enterococcus), when other options aren't suitable.
Why is the linezolid dose different for younger children — and why does mg/kg/day go down with age?
It's a clearance difference, not a dose reduction with age in the usual sense. A child under 12 metabolises and excretes linezolid faster per kg of body weight than an adolescent or adult, so the same 10 mg/kg-every-12-hours regimen used in older patients would leave a younger child under-dosed. Giving 10 mg/kg every 8 hours instead (30 mg/kg/day total) compensates for that faster clearance and produces roughly the same drug exposure as the lower 20 mg/kg/day adult regimen does in an adult's slower-clearing body. In short: the mg/kg/day number looks smaller in adults only because adults need less drug per kg to reach the same blood level that a child needs more drug per kg to reach — both are calibrated to hit the same therapeutic target, just via different starting metabolic rates.
Can linezolid be switched from IV to tablets at the same dose?
Yes — linezolid has near-complete oral bioavailability, so the oral dose is generally the same as the IV dose, allowing a straightforward switch once a patient can take oral medication.
Does linezolid interact with antidepressants?
Yes — it's a weak monoamine oxidase inhibitor and can cause serotonin syndrome when combined with SSRIs, SNRIs, or other serotonergic medications. Review the full medication list before prescribing.
References
- British National Formulary for Children (BNFc) — Linezolid.
- American Academy of Pediatrics Red Book — Linezolid.
- Liu C, et al. Clinical Practice Guidelines for MRSA Infections in Adults and Children (IDSA). Clin Infect Dis. 2011.