Co-amoxiclav is the generic name for amoxicillin combined with clavulanic acid — a prescription antibiotic sold as Augmentin, Ranclav, Myclav, Co Maug and others. It treats chest, sinus, ear, urinary, skin and dental infections, particularly where plain amoxicillin would fail.
What is Co Amoxiclav?
“Co-amoxiclav” is the name of the combination itself, not a brand — from co (combined), amox (amoxicillin) and clav (clavulanic acid). If your pack carries a brand name, co-amoxiclav is what is inside.
It is Schedule 4 — prescription-only — and comes in tablets and paediatric suspensions.
The numbers on the pack are the two ingredients added together, which causes constant confusion:
| Pack name | Amoxicillin | Clavulanic acid |
|---|---|---|
| 375 | 250 mg | 125 mg |
| 625 | 500 mg | 125 mg |
| 1 g | 875 mg | 125 mg |
So “625” does not mean 625 mg of antibiotic — it means 500 mg of amoxicillin plus 125 mg of clavulanic acid.
Only one of the two ingredients is an antibiotic. Amoxicillin is a penicillin that kills bacteria by preventing them building their cell walls. Clavulanic acid has almost no antibacterial activity of its own.
Its role is specific and rather elegant. Many bacteria produce beta-lactamase, an enzyme that destroys penicillins before they can act — the most common reason plain amoxicillin fails. Clavulanic acid binds and disables that enzyme, acting as a shield so the amoxicillin survives long enough to work.
Notice that the clavulanic acid amount stays at 125 mg while the amoxicillin rises. That is deliberate: 125 mg is enough to neutralise the enzyme, and more would only add to the stomach upset it causes. Two 625 tablets are therefore not equivalent to one 1 g tablet — the clavulanic acid would be doubled unnecessarily. This is exactly why you should not improvise doses from a different pack strength.
What is Co Amoxiclav commonly used for?
A doctor may prescribe it for:
- Chest infections, including some pneumonia and infective exacerbations of COPD.
- Sinusitis, where symptoms are persistent, severe, or worsening after initial improvement.
- Middle ear infections, particularly where amoxicillin has not worked.
- Urinary tract infections.
- Skin and soft-tissue infections, including infected animal and human bites — a classic use, as bite wounds carry mixed bacteria this combination covers well.
- Dental infections and abscesses, alongside dental treatment.
- Bone and joint infections, in some cases.
It has no effect on viruses. Colds, flu, most sore throats, most coughs and most bronchitis are viral, and an antibiotic will not shorten them or prevent complications.
It is a reserved rather than automatic first choice. Co-amoxiclav is broader-spectrum than plain amoxicillin — an advantage when resistance is likely, a disadvantage otherwise, since broader antibiotics disturb more of the body’s normal bacteria and drive more resistance.
How Co Amoxiclav fits into this medicine category
| Group | Examples | Typical role |
|---|---|---|
| Penicillins | Amoxicillin, flucloxacillin | Common first-line choices |
| Penicillin + inhibitor | Co-amoxiclav | Where beta-lactamase resistance is likely |
| Cephalosporins | Cefalexin | Skin, throat, urinary infections |
| Macrolides | Azithromycin | Chest infections, penicillin allergy |
| Fluoroquinolones | Ciprofloxacin | Urinary, gut and bone infections; reserved |
| Nitroimidazoles | Metronidazole | Anaerobic and some parasitic infections |
See the antibiotics and antimicrobials hub.
What to check before using Co Amoxiclav
Tell your doctor about any penicillin allergy before you start. This is the most important check. A previous rash, swelling or breathing difficulty on penicillin or amoxicillin must be known about, because co-amoxiclav is a penicillin and the reaction can be severe. Describe what actually happened — a mild rash years ago is a very different matter from swelling or difficulty breathing.
Take it with food or at the start of a meal. This meaningfully reduces the stomach upset and diarrhoea caused by the clavulanic acid, and improves absorption.
Complete the full course.
Tell your doctor or pharmacist if you:
- Have had any reaction to a penicillin or cephalosporin.
- Have had jaundice or liver problems on co-amoxiclav before. This is a specific contraindication — the liver reaction can recur more severely on re-exposure.
- Have liver or kidney disease.
- Have glandular fever (infectious mononucleosis). Amoxicillin commonly causes a widespread rash in this setting, and is usually avoided.
- Take warfarin, as co-amoxiclav can increase its effect.
- Take methotrexate or allopurinol.
- Are pregnant or breastfeeding.
- Use the combined oral contraceptive pill. Antibiotics do not generally reduce its effectiveness, but severe vomiting or diarrhoea can.
Shake suspensions well, use the measuring device supplied rather than a kitchen spoon, and check storage — most reconstituted suspensions must be refrigerated and discarded after a set number of days.
Possible cautions and when to ask a pharmacist or doctor
The most common effects are diarrhoea, nausea, stomach upset and thrush (oral or vaginal). Diarrhoea is notably more common than with plain amoxicillin, because of the clavulanic acid.
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.
- Severe, watery or bloody diarrhoea, during or in the weeks after the course. This may indicate a C. difficile infection, and anti-diarrhoeal medicines such as loperamide should not be used in that situation.
- Yellowing of the eyes or skin, dark urine, pale stools or itching — a possible liver reaction, which can appear up to several weeks after finishing the course. This is more common in older people and with longer courses.
- Unusual bruising or bleeding.
- Much less urine than usual.
- A seizure, which is rare and more likely with kidney impairment.
Contact your doctor if you are no better after two to three days, or if symptoms worsen at any point — the organism may not be susceptible.
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 co-amoxiclav used for? Bacterial infections of the chest, sinuses, ears, urinary tract, skin and teeth, particularly where resistance to plain amoxicillin is likely.
What does the name mean? It stands for combined amoxicillin and clavulanic acid. It is the generic name for the combination, sold under brands including Augmentin, Ranclav and Myclav.
Why does 625 not mean 625 mg of antibiotic? The number adds both ingredients: 500 mg amoxicillin plus 125 mg clavulanic acid.
Can I take two 625 tablets instead of one 1 g tablet? No. That would double the clavulanic acid unnecessarily, increasing stomach upset. Take the strength you were prescribed.
What does the clavulanic acid do? It blocks the bacterial enzyme that would otherwise destroy amoxicillin, restoring its effectiveness against resistant bacteria.
Will it help flu or a cold? No. Those are viral, and antibiotics have no effect on them.
Why does it upset my stomach? Mostly the clavulanic acid. Taking it with food or at the start of a meal reduces this considerably.
What if I get jaundice afterwards? See a doctor. A liver reaction can appear up to several weeks after finishing the course, and it must be recorded so you are not given it again.