Antibiotics treat bacterial infections — urinary infections, chest infections, skin abscesses, sexually transmitted infections, prostate infections and many others. They do nothing at all for viruses, which cause most coughs, colds and sore throats. Which antibiotic you need depends entirely on which bacteria and where.
What is Antibiotics?
An antibiotic is a medicine that kills bacteria or stops them multiplying, letting your immune system clear the infection.
Different classes attack bacteria in different ways:
- Penicillins (amoxicillin, co-amoxiclav) and cephalosporins break down the bacterial cell wall, so the cell bursts as it divides. Human cells have no cell wall, which is why these are so well tolerated.
- Macrolides (azithromycin, erythromycin) and tetracyclines (doxycycline) block bacterial protein manufacture.
- Quinolones (ciprofloxacin) interfere with bacterial DNA copying.
- Sulfonamide combinations (co-trimoxazole) starve bacteria of folate, which they must make themselves.
- Nitroimidazoles (metronidazole) damage DNA in bacteria that live without oxygen.
They are not interchangeable. Each class reaches different tissues and covers different organisms. An antibiotic that clears a urinary infection may be useless for a chest infection, and one that works in the bladder may not reach the prostate. This is why leftover antibiotics from a previous illness are not a shortcut — the wrong one does nothing except breed resistance and cause side effects.
Narrow-spectrum and broad-spectrum. A narrow antibiotic targets a limited range of bacteria; a broad one covers many. Broad sounds better and usually is not — it disturbs your normal gut bacteria more, causes more diarrhoea and thrush, and drives resistance faster. Good prescribing uses the narrowest antibiotic that will work.
Antibiotics do not work on viruses. Viruses replicate inside your own cells using your machinery, and have none of the structures antibiotics attack.
What is Antibiotics commonly used for?
Urinary tract infections (UTIs) — one of the commonest reasons for a prescription. Nitrofurantoin, co-trimoxazole, fosfomycin and others are used for simple cystitis; kidney infections need something that reaches the kidney. Get assessed rather than self-treating if you are male, pregnant, diabetic, have a catheter, have fever or back pain, or get UTIs repeatedly.
Chest infections — bacterial bronchitis and pneumonia. Most coughs are viral and need no antibiotic. Antibiotics are for a chest infection with a bacterial pattern: persistent fever, breathlessness, feeling genuinely unwell, or discoloured sputum with those.
Skin infections and abscesses. Cellulitis needs antibiotics. An abscess usually needs draining, and an antibiotic alone will often not settle it — this is the single most common misunderstanding about skin infections. A collection of pus needs to come out.
Sexually transmitted infections. Gonorrhoea, chlamydia, syphilis and others are bacterial and curable. South African public clinics treat STIs free and confidentially, and partners need treating too or reinfection follows. Herpes and HIV are viral and need different medicines entirely.
Prostate infections (prostatitis). These need an antibiotic that penetrates prostate tissue — commonly a quinolone or co-trimoxazole — and a much longer course than a bladder infection, often several weeks. Stopping early is a common cause of it becoming chronic.
Dental infections, alongside dental treatment. Ear and throat infections, where bacterial. Gut infections, some of them. Bone and joint infections. Wound and bite infections.
Prevention, in specific situations — before some surgery, and long term in a few conditions.
How Antibiotics fits into this medicine category
| Class | Examples | Commonly for |
|---|---|---|
| Penicillins | Amoxicillin, co-amoxiclav | Chest, ear, sinus, dental, skin, bites |
| Macrolides | Azithromycin, erythromycin | Chest, STIs, penicillin allergy |
| Cephalosporins | Cefuroxime, ceftriaxone | Broad cover, often in hospital |
| Tetracyclines | Doxycycline | Chest, acne, STIs, tick-bite fever |
| Quinolones | Ciprofloxacin | Reserved; UTIs, prostate |
| Nitrofurans | Nitrofurantoin | Simple bladder infections only |
| Sulfonamide combos | Co-trimoxazole | UTIs, chest, HIV-related prevention |
| Nitroimidazoles | Metronidazole | Anaerobic, dental, gut, some STIs |
Antibiotics are not antifungals or antivirals. Thrush needs an antifungal; flu, herpes and HIV need antivirals.
See the antibiotics and antimicrobials hub.
What to check before using Antibiotics
Tell your doctor about any antibiotic allergy, and what the reaction was. A rash is different from facial swelling or collapse, and the distinction changes what can safely be prescribed. Penicillin allergy is over-reported — many people labelled allergic in childhood are not, and being wrongly labelled means receiving broader, less effective antibiotics for life. It is worth having properly assessed.
Also tell them if you:
- Have kidney or liver disease — many doses need adjusting.
- Are pregnant or breastfeeding. Several classes are avoided.
- Take warfarin, methotrexate, statins, the contraceptive pill, or antacids and iron — all interact with one antibiotic class or another.
- Have had C. difficile diarrhoea before.
- Have myasthenia gravis, epilepsy, or tendon problems — relevant to specific classes.
- Are giving it to a child, where doses are by weight.
Four rules that matter more than any of the above:
Finish the course as prescribed. Feeling better on day three means it is working, not that the infection has gone.
Never use leftover antibiotics, or someone else’s. Wrong drug, wrong dose, wrong duration.
Do not push for antibiotics for a viral illness. Every unnecessary course makes the next infection harder to treat — for you and everyone else. Antibiotic resistance is not an abstract future problem; resistant infections already kill people in South African hospitals.
Take them as directed — with or without food, spaced evenly, and observing specific instructions such as avoiding alcohol with metronidazole or staying upright after doxycycline.
Possible cautions and when to ask a pharmacist or doctor
Common effects across most antibiotics: nausea, diarrhoea, stomach upset, and thrush — oral or vaginal — because the medicine disturbs normal bacteria as well as the infection.
Get emergency help for:
- Swelling of the face, lips or tongue, difficulty breathing, throat tightness, dizziness or collapse.
- A widespread blistering or peeling rash, or a rash with fever and swollen glands.
- Severe, watery or bloody diarrhoea with fever or cramps, during or up to two months after a course. This may be C. difficile and needs treatment — do not simply take an anti-diarrhoeal.
Seek prompt advice for:
- Any rash, however mild.
- Yellowing of the skin or eyes, dark urine or pale stools — possible liver injury, which can appear weeks after the course ends.
- Sudden tendon pain or swelling, particularly the Achilles, on a quinolone. Stop and get advice.
- Severe sunburn on normal exposure — several antibiotics increase sun sensitivity.
- Unusual bruising or bleeding, especially on warfarin.
- Being no better after two to three days, or getting worse.
See a doctor rather than self-treating if: you have fever with back or flank pain; a spreading red area on the skin; an abscess or a collection of pus; difficulty breathing; a UTI and you are male, pregnant or diabetic; infections that keep returning; or you think you may have an STI. The last is worth stating plainly: STI testing and treatment at a public clinic is free and confidential, and delaying it causes complications that are far harder to fix than the infection.
Related medicine guides
Frequently asked questions
What are antibiotics used for? Bacterial infections — urinary, chest, skin, dental, ear, bone, gut and sexually transmitted infections among them. They do nothing for viruses.
What antibiotics are used for UTI? Commonly nitrofurantoin, co-trimoxazole or fosfomycin for simple cystitis, and something that reaches the kidney for a kidney infection. The right choice depends on local resistance and on you — it needs a prescriber.
What antibiotics are used for chest infections? Usually amoxicillin, co-amoxiclav, a macrolide or doxycycline. Most coughs are viral and need none.
What antibiotics are used for a skin abscess? Antibiotics help the surrounding infection, but an abscess usually needs draining. Antibiotics alone often will not settle a collection of pus.
What antibiotics are used for STDs? It depends on the infection — different antibiotics for gonorrhoea, chlamydia and syphilis. Public clinics test and treat free and confidentially, and partners need treating too.
What antibiotics are used for prostate infection? Usually a quinolone or co-trimoxazole, which penetrate prostate tissue, and for considerably longer than a bladder infection — often several weeks.
Will antibiotics help my cold or flu? No. Those are viral.
Can I stop when I feel better? No. Complete the course, or the infection can return and resistance is encouraged.
Can I use leftover antibiotics? No. Wrong drug, wrong dose, wrong duration — and it drives resistance without treating anything.