Purbac DS contains sulfamethoxazole 800 mg with trimethoprim 160 mg — the combination known as co-trimoxazole, at double the standard tablet strength. It is used for urinary, chest and skin infections, and very widely in South Africa to prevent infections in people living with HIV.
What is Purbac Double Strength?
Purbac is Schedule 4 — prescription-only — and comes in several strengths:
| Product | Contains |
|---|---|
| Purbac Adult | Sulfamethoxazole 400 mg + trimethoprim 80 mg |
| Purbac DS | Sulfamethoxazole 800 mg + trimethoprim 160 mg |
| Purbac suspension | Sulfamethoxazole 200 mg/5 ml + trimethoprim 40 mg/5 ml |
“DS” means double strength — exactly twice the standard adult tablet. One DS tablet equals two regular tablets, which is worth confirming, since taking two DS tablets when two regular ones were intended doubles the dose.
The combination is called co-trimoxazole, always in a fixed 5:1 ratio of sulfamethoxazole to trimethoprim.
Why combine two antibiotics? Because they block two consecutive steps in the same pathway, and that is unusually effective.
Bacteria must manufacture their own folate — they cannot absorb it from their surroundings as human cells do. That pathway has two key steps:
- Sulfamethoxazole blocks the first step, competing with PABA.
- Trimethoprim blocks the second, inhibiting dihydrofolate reductase.
Blocking two sequential steps produces a synergistic effect — the combination kills bacteria that either alone would only slow. It also makes resistance far less likely, since a bacterium would need to overcome both.
Human cells are unaffected because we obtain folate from our diet rather than making it, and human dihydrofolate reductase is structurally different enough that trimethoprim barely touches it.
What is Purbac Double Strength commonly used for?
- Urinary tract infections, including recurrent and complicated ones.
- Chest infections, including some pneumonia and exacerbations of COPD.
- ***Pneumocystis jirovecii* pneumonia (PCP) — both treating it and, more commonly, preventing** it. This is a major use in South Africa.
- Preventing infections in people living with HIV. Co-trimoxazole preventive therapy is a cornerstone of HIV care, given to people with low CD4 counts and to many others, and it prevents PCP, toxoplasmosis, some bacterial infections and some malaria. It saves lives, and it should not be stopped without medical advice even when you feel well.
- Skin and soft-tissue infections, including some caused by MRSA.
- Ear and sinus infections.
- Traveller’s diarrhoea and some gut infections.
- Toxoplasmosis and nocardiosis.
- Prostate infections.
It has no effect on viruses. Colds, flu and most sore throats are viral.
How Purbac Double Strength fits into this medicine category
| Group | Examples | Typical role |
|---|---|---|
| Sulfonamide + trimethoprim | Co-trimoxazole (Purbac) | Urinary, chest, skin; PCP prevention |
| Penicillins | Amoxicillin, flucloxacillin | Common first-line choices |
| 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 Purbac Double Strength
Drink plenty of water while taking it. Sulfonamides can crystallise in the kidneys if you are dehydrated.
Take it with food to reduce stomach upset, and complete the full course.
If it was prescribed as preventive therapy for HIV, take it every day and do not stop because you feel well — that is exactly what it is preventing.
Tell your doctor or pharmacist if you:
- Have a sulfonamide (“sulfa”) allergy. This is the most important check — reactions to co-trimoxazole can be severe.
- Have kidney disease, which usually means a lower dose.
- Have liver disease.
- Have G6PD deficiency, where it can cause red blood cell breakdown.
- Have folate deficiency, or are malnourished.
- Have a blood disorder, or take medicines affecting the bone marrow.
- Have asthma or severe allergies.
- Have porphyria.
- Are elderly, where the risk of blood and skin reactions and of high potassium is greater.
- Are pregnant or breastfeeding. It is generally avoided in the first trimester and near term.
- Take warfarin — co-trimoxazole markedly increases its effect and bleeding risk. A significant and well-recognised interaction.
- Take methotrexate, where the combination can cause severe bone marrow suppression.
- Take ACE inhibitors, ARBs or potassium-sparing diuretics. Trimethoprim raises potassium, and the combination can push it to dangerous levels — particularly in older people.
- Take phenytoin, ciclosporin, digoxin or sulfonylureas for diabetes.
Possible cautions and when to ask a pharmacist or doctor
Common effects are nausea, vomiting, loss of appetite, diarrhoea and rash.
Stop and seek medical attention urgently if you develop:
- Any rash. Co-trimoxazole is among the medicines most associated with severe skin reactions — Stevens-Johnson syndrome and toxic epidermal necrolysis — which can be fatal. Do not wait to see whether it settles. Seek attention the same day, particularly with fever, blistering, peeling skin, or sores in the mouth, eyes or genitals.
- Swelling of the face, lips, tongue or throat, or difficulty breathing.
- Fever, sore throat, mouth ulcers, unusual bruising or bleeding, or extreme tiredness — possible bone marrow suppression.
- Yellowing of the eyes or skin, or dark urine.
- Much less urine than usual.
- Muscle weakness, palpitations or an irregular heartbeat — possible high potassium.
- Severe or bloody diarrhoea, during or after the course.
- Confusion, or unsteadiness.
- Joint or muscle pain with a rash and fever.
If you are on long-term preventive therapy, expect periodic blood tests for blood count, kidney function and potassium.
Contact your doctor if you are no better after two to three days of treatment, or if symptoms worsen.
Related medicine guides
- Browse Antibiotics, Antifungals & Antimicrobials guides
- Read: Azrotrin used for
- Read: Metronidazole used for
Frequently asked questions
What is Purbac Double Strength used for? Urinary, chest and skin infections, and very widely to prevent infections — particularly PCP — in people living with HIV. It contains sulfamethoxazole 800 mg with trimethoprim 160 mg.
What does “DS” mean? Double strength — exactly twice the standard adult tablet. One DS tablet equals two regular Purbac tablets.
Why does it contain two antibiotics? They block two consecutive steps in the bacterial folate pathway, which works better than either alone and makes resistance much less likely.
Why is it given in HIV care? Co-trimoxazole preventive therapy prevents PCP, toxoplasmosis and several bacterial infections. It is a cornerstone of HIV care and should not be stopped without medical advice.
Can I stop it if I feel well? Not if it was prescribed as preventive therapy — feeling well is what it is achieving. Speak to your doctor before stopping.
What if I get a rash? Stop and seek medical attention the same day. Co-trimoxazole is associated with rare but serious skin reactions.
Can I take it if I am allergic to sulfa drugs? Tell your doctor first. A sulfonamide allergy is the most important check with this medicine.
Does it interact with warfarin? Yes, significantly — it increases warfarin’s effect and bleeding risk. Your doctor must know.