How the dose is calculated
This calculator covers the two standard rifampicin contact-prophylaxis regimens, each with its own mg/kg dose, frequency, and course length. Rifampicin's role in multidrug tuberculosis treatment uses separate weight-band dosing tables as part of fixed-dose combination regimens and is not covered by this calculator.
Typical dosing by indication
| Indication | Dose | Frequency | Max |
|---|---|---|---|
| Meningococcal prophylaxis | 20 mg/kg/day | every 12h (2×/day) for 2 days | 1200 mg/day |
| Hib prophylaxis | 20 mg/kg once daily | once daily for 4 days | 600 mg/day |
Rifampicin is also a cornerstone of multidrug tuberculosis treatment, but TB dosing follows separate weight-band fixed-dose combination tables rather than a simple mg/kg/day calculation — this calculator covers contact-prophylaxis dosing only.
Clinical use
- Meningococcal contact prophylaxis is given to close contacts of a confirmed case as soon as possible, ideally within 24 hours of diagnosis, for the short 2-day course shown above.
- Haemophilus influenzae type b (Hib) contact prophylaxis follows a longer 4-day once-daily course, used for household contacts including an unvaccinated or incompletely vaccinated young child in the household.
- Rifampicin is a potent inducer of hepatic CYP450 enzymes, significantly reducing the effectiveness of many drugs including oral contraceptives, warfarin, and some antiretrovirals — review the full medication list before prescribing.
- Harmlessly turns urine, tears, sweat, and contact lenses orange-red — warn patients in advance to avoid alarm.
Frequently asked questions
What is rifampicin used for in this calculator?
This calculator covers rifampicin's use as short-course prophylaxis for close contacts of confirmed meningococcal disease or Haemophilus influenzae type b (Hib) infection — not its use as part of multidrug tuberculosis treatment, which follows separate dosing tables.
Why are the meningococcal and Hib prophylaxis courses different lengths?
They're separate, independently established regimens — meningococcal prophylaxis uses a twice-daily dose for 2 days, while Hib prophylaxis uses a once-daily dose for 4 days. Both are effective for their specific indication when given as designed.
Why does rifampicin turn urine and tears orange?
This is a harmless, expected effect of the drug's own colour being excreted in body fluids — it doesn't indicate liver injury or any other problem, but patients should be warned in advance so it isn't mistaken for something concerning.
Does rifampicin interact with the contraceptive pill?
Yes — rifampicin strongly induces liver enzymes that break down oestrogen and progestogen, significantly reducing the effectiveness of hormonal contraception. Patients should be advised to use an additional or alternative contraceptive method during and shortly after treatment.
References
- Centers for Disease Control and Prevention. Meningococcal Disease — Chemoprophylaxis Guidance.
- American Academy of Pediatrics Red Book — Haemophilus influenzae Infections.
- British National Formulary for Children (BNFc) — Rifampicin.