Septapen contains flucloxacillin 250 mg, a prescription penicillin antibiotic. It is used mainly for skin and soft-tissue infections caused by Staphylococcus — boils, cellulitis, infected wounds and infected eczema — and for some bone and chest infections. It must be taken on an empty stomach.
What is Septapen 250?
Septapen is Schedule 4 — prescription-only:
| Product | Contains |
|---|---|
| Septapen 250 | Flucloxacillin 250 mg tablet |
Flucloxacillin is available under other names in South Africa too — see the flucloxacillin guide. Being dispensed one rather than another is ordinary generic substitution.
Flucloxacillin is a penicillinase-resistant penicillin, and understanding that phrase explains everything about when it is used.
**Most Staphylococcus aureus produce an enzyme called penicillinase** (a beta-lactamase) that destroys ordinary penicillins before they can work. That is why plain penicillin and amoxicillin fail against most staph infections.
Flucloxacillin’s molecular structure physically shields the vulnerable part of the molecule from that enzyme. It survives, and kills the bacterium by blocking the enzymes that build its cell wall — so the wall fails and the bacterium bursts.
The trade-off is narrow coverage. Flucloxacillin is excellent against staph and streptococci and does relatively little against other organisms. That narrowness is a strength rather than a weakness when the likely organism is known: a targeted antibiotic disturbs less of the body’s normal bacteria and drives less resistance than a broad one.
Where the organism is uncertain, it is sometimes combined with amoxicillin — as in Macropen — so that both are covered.
Flucloxacillin does not work against MRSA — methicillin-resistant Staphylococcus aureus — which has a different resistance mechanism the structural shielding does not address.
What is Septapen 250 commonly used for?
A doctor may prescribe it for:
- Skin and soft-tissue infections — cellulitis, boils, abscesses, infected wounds, infected eczema. The main use.
- Impetigo, where it is widespread or not responding to topical treatment.
- Infected cuts, grazes and bites.
- Bone and joint infections, including osteomyelitis, often as a longer course.
- Chest infections caused by staph.
- Endocarditis, a heart valve infection, in hospital.
- Preventing infection around certain surgery.
It has no effect on viruses. Colds, flu and most sore throats are viral, and an antibiotic will not shorten them.
How Septapen 250 fits into this medicine category
| Group | Examples | Typical role |
|---|---|---|
| Anti-staphylococcal penicillins | Flucloxacillin (Septapen) | Skin and soft-tissue infections; staph |
| Broad-spectrum penicillins | Amoxicillin | Ear, chest, urinary, dental infections |
| Penicillin + inhibitor | Co-amoxiclav | Where resistance is likely |
| Cephalosporins | Cefalexin | Skin, throat, urinary infections |
| Macrolides | Azithromycin | Alternative in penicillin allergy |
| Nitroimidazoles | Metronidazole | Anaerobic and some parasitic infections |
| Topical antibiotics | Mupirocin | Small, localised skin infections |
Small, localised skin infections often respond to a topical antibiotic, while spreading infection needs an oral one — a cream cannot reach far enough. See the antibiotics and antimicrobials hub.
What to check before using Septapen 250
Take it on an empty stomach — usually an hour before food or two hours after. This is the most important practical instruction, because food substantially reduces flucloxacillin absorption. Taking it with meals is a common reason treatment seems not to work.
Tell your doctor about any penicillin allergy before you start, and describe what actually happened during the previous reaction.
Complete the full course, and space doses evenly through the day.
Tell your doctor or pharmacist if you:
- Have had any reaction to a penicillin or cephalosporin.
- Have had jaundice or liver problems on flucloxacillin before. This is a specific contraindication.
- Have liver disease.
- Have kidney disease.
- Are over 65, or are being prescribed a course longer than two weeks — both raise the risk of the liver reaction described below.
- Are pregnant or breastfeeding.
- Take warfarin, methotrexate or probenecid.
- Use the combined oral contraceptive pill. Antibiotics do not generally reduce its effectiveness, but severe vomiting or diarrhoea can.
Possible cautions and when to ask a pharmacist or doctor
Common effects are nausea, diarrhoea, stomach upset and thrush.
Stop and seek medical attention urgently if you develop:
- Swelling of the face, lips, tongue or throat, difficulty breathing, or a widespread hives-like rash — a serious allergic reaction. Call emergency services.
- Any rash, which should be assessed rather than ignored.
- Blistering or peeling skin, or sores in the mouth or eyes.
- Yellowing of the eyes or skin, dark urine, pale stools or itching. This is the reaction most specific to flucloxacillin — a cholestatic jaundice that is uncommon but well recognised, more likely in older people and with courses over two weeks, and which can appear up to two months after finishing the course. Report it even if you stopped the antibiotic weeks ago, as it must be recorded so you are never given it again.
- Severe, watery or bloody diarrhoea, during or in the weeks after the course — possible C. difficile infection, where anti-diarrhoeal medicines should not be used.
- Fever, sore throat or mouth ulcers, or unusual bruising or bleeding.
- Much less urine than usual.
- Muscle aches or joint pain with a rash.
For a skin infection, watch the edge of the redness. Seek medical attention promptly rather than waiting to finish the course if:
- Redness is spreading, particularly with a defined advancing edge.
- Red streaks run from the area.
- Pain, swelling or warmth is increasing.
- You develop fever or feel generally unwell.
- There is pus, or a rapidly enlarging area.
- No improvement after two to three days.
- The infection is on the face, near the eye, or over a joint.
- You have diabetes or a weakened immune system, or the infection is on the foot — use a lower threshold in these cases.
Never use antibiotics left over from a previous illness or prescribed for someone else.
Related medicine guides
- Browse Antibiotics, Antifungals & Antimicrobials guides
- Read: Azrotrin used for
- Read: Metronidazole used for
Frequently asked questions
What is Septapen 250 used for? Bacterial infections of the skin and soft tissue — cellulitis, boils, infected wounds and infected eczema — and some bone and chest infections. It contains flucloxacillin 250 mg.
Why is it used mainly for skin infections? Most skin infections are caused by Staphylococcus aureus, and flucloxacillin resists the enzyme that staph use to destroy ordinary penicillins.
Should I take it with food? No — take it on an empty stomach, about an hour before or two hours after eating. Food substantially reduces absorption.
Does it work against MRSA? No. MRSA has a different resistance mechanism that flucloxacillin does not overcome.
Will it help flu or a cold? No. Those are viral, and antibiotics have no effect on them.
Can I take it if I am allergic to penicillin? Tell your doctor first — flucloxacillin is a penicillin. Describe exactly what happened during the previous reaction.
What if I turn yellow after finishing the course? See a doctor. Flucloxacillin can cause a liver reaction appearing up to two months later, and it must be recorded so you are not given it again.
When should I see a doctor about a skin infection? If redness is spreading, there are red streaks, you have fever, or there is no improvement after two to three days.