Skip to content
ClinCalc Pro
Menu
Calculators Drugs Pathways AI Dx Donate Get the App
Antibiotic — Penicillinase-Resistant Pregnancy: Animal studies with flucloxacillin have shown no teratogenic effects. The product has been in clinical use since 1970 and the limited number of reported cases of use in human pregnancy have shown no evidence of untoward effects. The decision to administer any drug during pregnancy should be taken with the utmost care. Therefore flucloxacillin should only be used in pregnancy when the potential benefits outweigh the potential risks associated with treatment. Lactation: Trace quantities of flucloxacillin can be detected in breast milk. The possibility of hypersensitivity reactions must be considered in breast-feeding infants. Therefore flucloxacillin should only be administered to a breast-feeding mother when the potential benefits outweigh the potential risks associated with the treatment.

Flucloxacillin

Brand names: Floxapen

Used in: Cellulitis & Skin Infection

Flucloxacillin is a penicillinase-resistant penicillin used for staphylococcal and streptococcal infections, including skin and soft-tissue infections.

Adult dose

Dose: Adults (including elderly patients), oral — infections due to sensitive Gram-positive organisms, including the skin and soft tissue and ear infections relevant to ENT practice (cellulitis, infected wounds, otitis media and externa, sinusitis, tonsillitis, pharyngitis, quinsy): 250 mg four times a day. In serious infections the dosage may be doubled.
Route: Oral (capsules, 250 mg)
Frequency: Four times a day, on an empty stomach (an hour before food or two hours after food)
The pinned product 'Flucloxacillin 250 mg capsules, hard' §4.1 lists "Otitis media and externa", sinusitis, tonsillitis, pharyngitis, quinsy, cellulitis, infected wounds and infected skin conditions, so ENT use is covered and a dose is written. The label prints no ENT-specific or orbital-specific regimen: periorbital or orbital cellulitis is covered only as "Cellulitis", and orbital or other severe infections needing intravenous treatment are outside this oral card. ROUTE: this card covers the ORAL capsule only. The page's intravenous route is NOT covered by this product and no IV dose is written. NOT CARRIED INTO THE ORAL CARD DOSE (printed in the same posology but with no route stated, or for IV use): osteomyelitis, endocarditis — "Up to 8 g daily, in divided doses six to eight hourly"; surgical prophylaxis — "1 to 2 g IV at induction of anaesthesia followed by 500 mg six hourly IV, IM or orally for up to 72 hours". MAXIMUM: no maximum is printed for the oral regimen; the sentence "The maximum recommended dose in adults is 1 g every 8 to 12 hours" sits in the renal-function paragraph and is recorded under renalAdjustment, not here. NO DURATION is printed in the retrieved §4.2. The dosage "depends on the age, weight and renal function of the patient, as well as the severity of the infection". ADMINISTRATION: "This medicine should be taken on an empty stomach. This means an hour before food or two hours after food." "The capsules should be taken with a full glass of water (250 ml), to reduce the risk of oesophageal pain"; patients should not lay down immediately after taking it. "Parenteral usage is indicated where oral dosage is inappropriate." Hepatotoxicity: hepatitis and cholestatic jaundice may be delayed for up to two months post-treatment; caution in patients over 50 years or with hepatic dysfunction (§4.4).

Paediatric dose

Dose: 2-10 years: 125 mg four times daily; under 2 years: 62.5 mg four times daily.
Route: Oral
Frequency: Four times daily
These are the label's own printed paediatric figures (not scaled from the adult dose). The label prints no figure for children over 10 years, and for "Premature infants, neonates, sucklings and infants" it says only "Other pharmaceutical forms/strengths may be more appropriate for administration to this population." No paediatric maximum is printed. The pinned product is a 250 mg capsule, so 125 mg and 62.5 mg doses need another presentation. §4.4: "special caution is essential in the newborn because of the risk of hyperbilirubinaemia" and reduced renal excretion. Use a children's formulary for any figure not printed here.

Dose adjustments

Renal

SPC §4.2: "In common with other penicillins, Flucloxacillin usage in patients with renal impairment does not usually require dosage reduction. However, in the presence of severe renal failure (creatinine clearance < 10 ml/min) a reduction in dose or an extension of dose interval should be considered." "Flucloxacillin is not significantly removed by dialysis and hence no supplementary dosages need to be administered either during, or at the end of the dialysis period." The same paragraph ends with the sentence "The maximum recommended dose in adults is 1 g every 8 to 12 hours." The SPC does not say to whom that sentence applies (it follows the renal-failure text, yet the same posology allows up to 8 g daily for osteomyelitis or endocarditis), so it is recorded here and NOT used as the maxDose for the oral regimen. §4.4 gives the reason for reduction in severe renal failure as the risk of neurotoxicity.

Hepatic

SPC §4.2: "Dose reduction in patients with reduced hepatic function is not necessary." §4.3: contra-indicated in patients with a previous history of flucloxacillin-associated jaundice/hepatic dysfunction. §4.4: "Flucloxacillin should be used with caution in patients with evidence of hepatic dysfunction, patients >50 years or patients with underlying disease all of whom are at increased risk of hepatic reactions."

Dose independently re-sourced from UK Summary of Product Characteristics (SPC) via the eMC — clinician-reviewed. Always confirm against the product SmPC and your local formulary before prescribing.

Contraindications

  • Hypersensitivity to the active substance, to any excipients listed in section 6.1, or to beta-lactam antibiotics (e.g. penicillins, cephalosporins) (§4.3).
  • Previous history of flucloxacillin-associated jaundice/hepatic dysfunction (§4.3).
  • Acute generalised exanthematous pustulosis (AGEP) on flucloxacillin: discontinue, and any subsequent administration is contra-indicated (§4.4).

Side effects

  • Common: minor gastrointestinal disturbances (§4.8).
  • Uncommon: rash, urticaria and purpura (§4.8).
  • Very rare: neutropenia (including agranulocytosis) and thrombocytopenia (reversible on discontinuation), eosinophilia, haemolytic anaemia (§4.8).
  • Very rare: anaphylactic shock (exceptional with oral administration), angioneurotic oedema; if any hypersensitivity reaction occurs, discontinue treatment (§4.8).
  • Very rare: pseudomembranous colitis (discontinue and give appropriate therapy, e.g. oral vancomycin) (§4.8). Not known: oesophageal pain and related events (§4.8).
  • Very rare: hepatitis and cholestatic jaundice (may be delayed for up to two months post-treatment and protracted for months; not related to dose or route; rarely fatal, most deaths in patients aged 50 or over or with serious underlying disease), changes in liver function tests. Risk is increased in carriers of the HLA-B*5701 allele, but routine screening is not recommended (§4.4, §4.8).
  • Very rare: erythema multiforme, Stevens-Johnson syndrome and toxic epidermal necrolysis. Not known: AGEP (§4.8).
  • Very rare: high anion gap metabolic acidosis when used with paracetamol, generally in the presence of risk factors (§4.4, §4.8). Not known: hypokalaemia, potentially life threatening and can be resistant to potassium supplementation, especially at high doses (§4.4, §4.8).
  • §4.8 is truncated at the source-fetch limit (musculoskeletal and later system-organ classes not retrieved).

Interactions

  • Probenecid and sulfinpyrazone slow excretion of flucloxacillin by decreasing tubular secretion; other drugs excreted via renal tubular secretion, such as piperacillin, may interfere with flucloxacillin elimination (§4.5).
  • Oral typhoid vaccine may be inactivated by flucloxacillin (§4.5).
  • Methotrexate: flucloxacillin reduces its excretion, which can cause methotrexate toxicity (§4.5).
  • Sugammadex: flucloxacillin may reduce the response (§4.5).
  • Warfarin: rare cases of altered INR; monitor prothrombin time or INR carefully during addition or withdrawal of flucloxacillin (§4.5).
  • Bacteriostatic drugs may interfere with the bactericidal action of flucloxacillin (§4.5).
  • Paracetamol: concurrent use associated with high anion gap metabolic acidosis (HAGMA), especially in patients with risk factors such as severe renal impairment, sepsis or malnutrition; monitor acid-base status, including urinary 5-oxoproline (§4.4, §4.5).
  • Hypokalaemia-inducing diuretics and other risk factors for hypokalaemia: attention warranted with flucloxacillin, especially at higher doses (§4.4).
  • The §4.5 list is cut off at the source-fetch limit at the start of the statement on flucloxacillin as a CYP450 inducer.

Monitoring

  • During prolonged treatment (e.g. osteomyelitis, endocarditis), regular monitoring of hepatic and renal function is recommended (§4.4).
  • Hepatic effects can begin up to two months after treatment ends and may be protracted (§4.4).
  • Serum potassium: regular measurement recommended with higher doses of flucloxacillin (§4.4).
  • INR / prothrombin time with warfarin during addition or withdrawal of flucloxacillin (§4.5).
  • After co-administration with paracetamol, close monitoring for acid-base disorders (HAGMA), including the search for urinary 5-oxoproline (§4.4).
  • Watch for hypersensitivity reactions and, at treatment initiation, a feverish generalised erythema with pustules (possible AGEP) (§4.4).

Clinical monograph

How it works

A beta-lactam, stable to staphylococcal penicillinase, that inhibits bacterial cell-wall synthesis.

Prescribing in practice

  • Contraindicated in penicillin allergy and in previous flucloxacillin-associated jaundice or hepatic dysfunction.
  • Cholestatic hepatitis can occur, sometimes up to several weeks after stopping, and is more common in older patients and with prolonged courses.
  • Take it on an empty stomach for better absorption.

Monitoring

Consider liver function with prolonged courses; review clinical response.

Counselling the patient

  • Take it before food and complete the course.
  • Report yellowing of the skin or eyes or dark urine, even after finishing the course.

Evidence & guidelines

First-line for staphylococcal skin and soft-tissue infection (e.g. cellulitis) per NICE/PHE guidance.

Reference: RCOphth Orbital Cellulitis Guidelines; MHRA Drug Safety Update (cholestatic jaundice); Drug verified in RxNorm (NLM); confirm dosing against the manufacturer SPC (eMC). Verify against your local formulary and current prescribing references before prescribing. The structured dose values shown have been reviewed by a clinician. Monograph status: clinician-reviewed (2026-07-04).

Related

Curated clinical cross-links plus same-class fallbacks.

Calculators
Pathways