Flucloxacillin is a penicillin antibiotic used for skin and soft tissue infections caused by Staphylococcus — boils, abscesses, cellulitis and infected wounds — and for bone and joint infections. It must be taken on an empty stomach, and liver effects can begin weeks after the course ends.
What is Flucloxacillin?
Flucloxacillin is a narrow-spectrum penicillin, dispensed as 250 mg and 500 mg capsules and as a suspension. It is a Schedule 4 prescription medicine, sold under its own name and as Indo Flucloxacillin, Staphylex and Floxapen.
It kills bacteria the way all penicillins do — by preventing them from building their cell walls, so the bacterium bursts. Human cells have no cell wall, which is the basis of its selectivity.
What sets flucloxacillin apart is a specific piece of chemistry. Many strains of Staphylococcus aureus — the organism behind most skin infections — produce an enzyme called penicillinase, which destroys ordinary penicillin before it can act. Flucloxacillin’s molecular structure shields it from that enzyme.
It is a penicillin that staphylococci cannot disable, which is exactly why it is chosen for skin infections while amoxicillin is not.
The trade-off is that its range is narrow: excellent against staphylococci and streptococci, and very little against anything else. That narrowness is a virtue — a targeted antibiotic disturbs less of your normal bacteria and applies less pressure toward resistance.
What is Flucloxacillin commonly used for?
- Cellulitis, a spreading infection of the skin and the tissue beneath it.
- Boils, carbuncles and abscesses.
- Infected wounds and infected surgical sites.
- Impetigo, where widespread or not responding to topical treatment such as Fucidin.
- Infected eczema.
- Bone infection (osteomyelitis) and joint infection (septic arthritis), usually starting intravenously.
- Some chest infections where a staphylococcal cause is suspected.
- Endocarditis, an infection of the heart valves, in hospital.
An antibiotic alone often will not clear an abscess. Antibiotics reach bacteria through the bloodstream, and the inside of an abscess has little blood supply. Drainage is usually the definitive treatment, with flucloxacillin covering the surrounding tissue. If a boil or abscess is not settling on antibiotics, that is generally the reason — and it needs a doctor rather than a longer course.
For cellulitis, watch the edge of the redness. It should stop spreading within a couple of days of starting treatment. Marking the border with a pen is a genuinely useful way to tell. Continued spread, increasing pain, or fever means it needs review promptly.
How Flucloxacillin fits into this medicine category
| Group | Examples | Common targets |
|---|---|---|
| Anti-staphylococcal penicillins | Flucloxacillin | Skin, soft tissue, bone — staphylococci |
| Broad-spectrum penicillins | Amoxicillin, amoxicillin-clavulanate (Auro-Amoxiclav) | Chest, ear, throat, dental, urinary |
| Narrow-spectrum penicillins | Penicillin V (Betapen) | Streptococcal throat |
| Macrolides | Azithromycin, erythromycin | Used in penicillin allergy |
| Nitroimidazoles | Metronidazole | Anaerobic and dental infections |
If you are allergic to penicillin, flucloxacillin is not an option and a different class is used. See the antibiotics and antimicrobials hub.
What to check before using Flucloxacillin
Take it on an empty stomach — an hour before food or two hours after — with a full glass of water. This matters more than with most antibiotics: food substantially reduces how much flucloxacillin is absorbed, and taking it with meals is a common reason a course underperforms.
Finish the whole course, even once the infection looks better.
Tell your prescriber or pharmacist if:
- You are allergic to penicillin or any beta-lactam antibiotic. This is the critical question — reactions can be severe.
- You have liver disease, or have had jaundice after taking flucloxacillin before. If so, it should not be used again.
- You have kidney disease.
- You are pregnant or breastfeeding.
- You are over 65 or facing a prolonged course, where liver effects are more likely.
- You take methotrexate, warfarin, or the contraceptive pill — vomiting or diarrhoea during a course can reduce its reliability.
A four-times-a-day schedule on an empty stomach is genuinely awkward, and it is the main reason courses go unfinished. Ask your pharmacist to help you plan the timing around your meals before you leave.
Follow the dispensing label. This page does not set doses.
Possible cautions and when to ask a pharmacist or doctor
Liver effects deserve specific mention. Flucloxacillin can cause cholestatic hepatitis — a liver reaction that is uncommon but well documented with this particular antibiotic. It is more likely with courses beyond two weeks and in people over 65, and, unusually, it can begin up to several weeks after the course has finished.
Report yellowing of the skin or eyes, dark urine, pale stools, persistent itching, or unexplained nausea and abdominal pain — even after you have stopped taking it. People often do not connect symptoms weeks later to a finished course, which is precisely why this is worth knowing.
Allergic reactions. Penicillin allergy ranges from a rash to anaphylaxis. Seek emergency care for swelling of the face, lips or tongue, difficulty breathing, or a widespread rash with faintness.
Diarrhoea is common. Severe, watery or bloody diarrhoea, particularly with fever, can indicate Clostridioides difficile infection and needs medical attention — during or after the course.
Thrush may follow, as the antibiotic disturbs normal bacteria.
Watch the infection itself. Improvement should be visible within two to three days. Spreading redness, worsening pain, fever, or red streaking away from the area needs urgent review.
Seek urgent medical attention for facial or throat swelling, difficulty breathing, a widespread rash, yellowing of the skin or eyes, dark urine with pale stools, severe or bloody diarrhoea, high fever, or an infection spreading despite treatment.
Related medicine guides
Frequently asked questions
What is flucloxacillin used for? Skin and soft tissue infections — cellulitis, boils, abscesses, infected wounds — and bone and joint infections.
Why must I take it on an empty stomach? Food substantially reduces absorption. Take it an hour before eating or two hours after, with water.
How is it different from amoxicillin? Flucloxacillin resists the enzyme staphylococci use to destroy ordinary penicillin, which is why it is chosen for skin infections. Amoxicillin covers a broader range but not resistant staphylococci.
Can I take it if I am allergic to penicillin? No. Flucloxacillin is a penicillin. Tell your prescriber so a different class can be used.
My boil is not improving. Why? An antibiotic often cannot clear a collection of pus on its own, because there is little blood supply inside an abscess. Drainage is usually needed.
I finished the course weeks ago and now feel unwell. Could it be related? Possibly. Flucloxacillin can cause a liver reaction beginning weeks after the course ends. Report jaundice, dark urine, pale stools or persistent itching.