Septrin in Spain: Co-trimoxazole Antibiotic Explained
Medically Reviewed by Dr. Pedro Yelamo Vera, MD
Licensed Physician No. 484814280 · Official Medical College of Bizkaia (Spain)Last reviewed: 29 October 2026
Septrin is co-trimoxazole, the antibiotic sold as Bactrim in the US. What it treats in Spain, why it needs a prescription and which warning signs matter.
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Septrin is the Spanish brand of co-trimoxazole, an antibiotic that combines sulfamethoxazole and trimethoprim. It treats some bacterial infections, such as certain urine, ear and chest infections, and prevents some lung infections in people with weak immunity. In Spain it needs a prescription. In the US the same medicine is sold as Bactrim.
- Septrin (co-trimoxazole) is a prescription antibiotic in Spain: Septrin, Septrin Forte and Septrin Pediátrico.
- For bladder infections it is not usually the first choice; doctors keep it for when other antibiotics do not suit.
- It contains a sulfa drug. Do not take it if you are allergic to sulfonamides.
- A rash, especially with blisters, mouth sores or fever, means stop it and get urgent care.
Dr. Pedro Yelamo Vera is a GP licensed in Spain (Colegiado No. 484814280).
For a medical emergency, call 112 or go to the nearest local emergency department. Do not wait for a video consultation or an online reply.
What is Septrin used for?
Septrin contains two antibiotics that work together. Sulfamethoxazole is a sulfonamide, or "sulfa" drug. Trimethoprim blocks the same pathway one step later. Together they stop bacteria from making folate, which they need to grow. The Spanish product information lists:
- Main uses: treating and preventing Pneumocystis pneumonia, and preventing toxoplasmosis, nocardiosis and melioidosis. These matter most for people with weak immunity, such as after a transplant or with HIV.
- When first-choice antibiotics do not suit: uncomplicated urine infections, middle-ear infections, flare-ups of chronic bronchitis, infectious diarrhoea, and a few rarer infections.
It does nothing for colds, flu or other viruses. It is also not used for strep throat.
Is Septrin the same as Bactrim?
Yes. Septrin, Bactrim and Septra are all brand names for co-trimoxazole. The packs and strengths vary by country.
| Name | What it is | Where |
|---|---|---|
| Septrin, Septrin Forte, Septrin Pediátrico | Co-trimoxazole tablets and children's liquid | Spain (prescription) |
| Co-trimoxazole, Septrin | Same medicine | UK, Ireland (prescription) |
| Bactrim, Bactrim DS, Septra | Same medicine (sulfamethoxazole and trimethoprim) | US (prescription) |
| Trimethoprim alone | Only one of the two parts, used for urine infections in the UK | UK |
The Spanish packs come in different strengths for adults and children. The doctor and the leaflet set how much to take and for how long. Finish the course as prescribed.
Is Septrin used for a UTI in Spain?
Sometimes, but rarely first. Many urine bacteria in Spain are now resistant to co-trimoxazole. For a simple bladder infection in a woman, doctors in Spain often start with fosfomycin, which we explain in our Monurol guide. Septrin may be chosen when a urine culture shows it will work, or when other options do not suit you. Our UTI page explains how a doctor decides.
Do you need a prescription for Septrin in Spain?
Yes. Every Septrin product is listed as a medicamento sujeto a prescripción médica on the AEMPS CIMA database. All antibiotics need a prescription in Spain, and a pharmacy will not hand them over without one. We explain why in our guide to antibiotics in Spain.
Burning when you pee, or an infection that came back after antibiotics?
You can book a video visit with a GP licensed in Spain, in English or Spanish. He checks your symptoms and past results and tells you whether an antibiotic is needed.
Who should not take Septrin?
- People allergic to sulfa drugs, trimethoprim or co-trimoxazole.
- Babies under 6 weeks old, including premature babies.
- People with acute porphyria, a rare blood condition.
- Anyone taking dofetilide, a heart rhythm medicine.
Tell the doctor first if you have kidney or liver disease, G6PD deficiency, low folate, a blood disorder or PKU. Older people are more prone to serious side effects.
Pregnancy and breastfeeding: the leaflet says it should generally be avoided in pregnancy, unless the benefit clearly outweighs the risk. It should be avoided near the end of pregnancy. It passes into breast milk, so ask the doctor before using it while breastfeeding.
Which medicines interact with Septrin?
- Methotrexate: Septrin raises its levels and its effect on folate.
- Warfarin: Septrin makes it stronger, so blood tests need watching. Mention any blood thinner, including Sintrom.
- Phenytoin and digoxin: their levels can rise.
- Ciclosporin after a transplant, and diabetes tablets such as sulfonylureas (blood sugar can drop).
- Medicines that raise potassium, and water tablets in older people.
What Septrin does not treat
- Colds, flu, Covid and most sore throats, which are viral.
- Strep throat: the leaflet says it should not be used for it.
- A kidney infection with high fever or vomiting: that needs an urgent in-person check.
Side effects and when to stop
Common side effects are nausea, diarrhoea, headache, a mild rash and thrush. Raised potassium in the blood is very common, mainly in older people and with kidney problems. Septrin can also make skin burn more easily in the sun. Rarely it lowers blood cells or causes serious skin reactions.
Stop taking it and get medical help the same day if you have:
- A rash, especially with blisters, peeling skin, mouth or eye sores, or flu-like feelings.
- Severe or lasting diarrhoea, even weeks after the course.
- Easy bruising, bleeding, a sore throat with fever, or yellow skin or eyes.
- A new or worse cough or breathlessness.
Call 112 if you have swelling of the face or throat, trouble breathing, or fever with very low blood pressure, a racing heart or confusion.
Talking it through with a doctor by video
Dr. Yelamo is a GP licensed in Spain. He sees adults, children and pregnant patients by video, every day from 07:30 to 20:30 (Spain time). In the visit he will:
- Ask about your symptoms, past infections and any urine culture results.
- Check for allergies, pregnancy and medicines that clash with Septrin.
- Explain whether an antibiotic is needed and which one suits you.
- Tell you which signs mean you need an exam in person.
He does not promise any test or prescription before the visit. Where clinically appropriate, following a genuine evaluation. Treatment and prescriptions are issued solely at the doctor's discretion based on evidence-based clinical guidelines.
A prescription is never guaranteed: it is issued only where clinically appropriate after a genuine evaluation, as a private electronic prescription under Spanish law. This service is for urgent, non-emergency care. In an emergency, call 112.
See how the Urgent Care video visit works. It lasts 15 minutes and costs €50.
This guide is general information. It does not replace the package leaflet or the advice of your doctor or pharmacist.
Frequently asked questions
What is Septrin?
The Spanish brand of co-trimoxazole, an antibiotic made of sulfamethoxazole and trimethoprim. It needs a prescription.
Is Septrin the same as Bactrim?
Yes. Bactrim and Septra are US brands of the same medicine, co-trimoxazole.
Is co-trimoxazole a sulfa drug?
Yes. Sulfamethoxazole is a sulfonamide. If you are allergic to sulfa drugs, tell the doctor and do not take it.
Is Septrin good for a bladder infection?
It can work, but resistance is common in Spain. Doctors often start with fosfomycin and keep Septrin for when it suits better.
What should I do if I get a rash on Septrin?
Stop taking it and get medical help the same day. Call 112 if your face or throat swells or you struggle to breathe.
Can I take Septrin while pregnant?
It is generally avoided in pregnancy, unless a doctor judges the benefit clearly outweighs the risk. Always tell the doctor if you are pregnant.
Sources
- AEMPS CIMA: Septrin Forte package leaflet
- MedlinePlus: Co-trimoxazole
- NHS: Urinary tract infections
- ECDC: Antimicrobial resistance
Related guides
Medical Citations & Sources
This content is written and reviewed by a licensed physician with reference to the following official health authorities.
