Flucloxacillin is an antibiotic used to treat bacterial skin and soft tissue infections, particularly those caused by Staphylococcus aureus. It belongs to the penicillin family and is specifically designed to work against staph bacteria that have developed resistance to older penicillins. It’s a first-line treatment for conditions like cellulitis, wound infections, and abscesses in the UK, Australia, and much of Europe (though it’s not available in the United States, where similar drugs like dicloxacillin are used instead).
Conditions It Treats
Flucloxacillin is a narrow-spectrum antibiotic, meaning it targets a specific group of bacteria rather than working broadly. Its main strength is against staph bacteria, including strains that produce an enzyme called penicillinase, which normally breaks down standard penicillin before it can work. Flucloxacillin is chemically modified to resist that enzyme.
The most common conditions treated with flucloxacillin include:
- Cellulitis and erysipelas: bacterial skin infections that cause redness, swelling, and warmth, often on the legs or face
- Wound infections: including surgical site infections and infected cuts or scrapes
- Abscesses: pockets of pus beneath the skin
- Impetigo: a highly contagious skin infection common in children
- Bone infections (osteomyelitis): deeper infections requiring longer courses of treatment
- Joint infections: particularly when caused by staph bacteria
It’s important to note that flucloxacillin does not work against MRSA (methicillin-resistant Staphylococcus aureus). It’s effective against methicillin-sensitive staph (MSSA), streptococci, and some other gram-positive bacteria.
How It Works
Like all penicillin-type antibiotics, flucloxacillin kills bacteria by interfering with their ability to build cell walls. Without intact cell walls, bacteria can’t survive or reproduce. The key difference between flucloxacillin and older penicillins is its resistance to the penicillinase enzyme. Many staph bacteria produce this enzyme specifically to neutralize penicillin, but flucloxacillin’s chemical structure protects it from being broken down.
Its effectiveness depends on maintaining a steady level of the drug in your bloodstream throughout the day. This is why it’s typically prescribed as multiple doses spread evenly across the day rather than a single large dose.
Typical Treatment Courses
For cellulitis, NICE guidelines recommend a 7-day course at 500 mg taken four times a day for adults. Children receive lower doses based on age: 62.5 mg to 125 mg four times daily for infants, scaling up to 250 to 500 mg four times daily for teenagers. Your doctor may adjust the dose or duration depending on how severe the infection is and how quickly it responds.
Deeper infections require much longer treatment. Bone infections, for example, often start with intravenous flucloxacillin for about two weeks before switching to oral capsules for several more weeks. The total course for osteomyelitis can stretch well beyond a month.
For more severe infections or when you can’t take pills, flucloxacillin can be given intravenously at higher doses, up to 2 g four times daily.
Taking It on an Empty Stomach
You’ll likely see instructions to take flucloxacillin on an empty stomach, at least 30 to 60 minutes before eating or two hours after a meal. This advice dates back to studies from the 1970s showing that food reduced peak blood levels of the drug by roughly 40% and delayed absorption.
More recent research has complicated this picture. A study in healthy volunteers found that while food does reduce overall drug levels, the concentrations that actually matter for killing bacteria were still reached in most circumstances, even when taken with food. So if taking it on an empty stomach causes nausea or makes you skip doses, taking it with a light meal is likely better than not taking it at all. That said, for serious infections where every advantage counts, taking it in a fasting state remains the most reliable approach.
Common Side Effects
The most frequent side effects are digestive: nausea, diarrhea, and stomach discomfort. These are typical of penicillin-type antibiotics and usually mild. Skin rashes can also occur, as with other penicillins.
The more serious concern with flucloxacillin is a specific type of liver injury called cholestatic jaundice. This causes painless yellowing of the skin and eyes, typically appearing 2 to 6 weeks after starting the drug, and sometimes up to 3 weeks after stopping it. Most people who develop this recover within several months, but in rare cases the damage can become chronic or, very rarely, fatal.
The risk of liver injury increases significantly with age. People over 60 are roughly six times more likely to develop cholestatic liver disease from flucloxacillin than younger patients. Female sex and treatment courses longer than 14 days have also been suggested as risk factors, though the evidence for those is less clear. If you notice yellowing of your skin or the whites of your eyes, dark urine, or persistent nausea during or after a course of flucloxacillin, contact your doctor promptly.
Who Should Not Take It
You should not take flucloxacillin if you’ve ever had an allergic reaction to it, to any other penicillin-type antibiotic, or if you have a history of liver problems linked to flucloxacillin use. Because it belongs to the penicillin family, a true penicillin allergy (especially one involving swelling, difficulty breathing, or anaphylaxis) rules it out. Make sure your doctor or pharmacist knows about any previous antibiotic reactions before starting a course.