Display Patient Information Leaflets (2023)

Date issued: August 2021

Review date: August 2023

Ref: F-65

PDF: Co-Trimoxazole (Septrin) final August 2021.pdf [pdf] 210KB

This leaflet will provide information for adults who have been offered Co-Trimoxazole (Septrin).

What is Co-Trimoxazole?

Co-Trimoxazole (sometimes called Septrin) is a type of antibiotic. An antibiotic is a type of medication used to treat infection. Co-Trimoxazole contains two separate antibiotics called trimethoprim and sulfamethoxazole.

What is Co-Trimoxazole used for?

Co-Trimoxazole can be used to treat infections including:

  • Bacterial infections of the lungs and airways. These infections are caused by types of microbes (germs) known as bacteria. They include infections such as pneumonia

  • Infections associated with other lung conditions like COPD and Cystic fibrosis. These are chronic lung conditions which can sometimes make your lungs more prone to infections

  • Infections in other parts of the body

  • A specific kind of pneumonia known as PCP. PCP is a type of fungus which can infect the lungs. It is also known as Pneumocystis jiroveci

Co-trimoxazole can also be used to help prevent infections. This is known as prophylactic use.

You may be offered prophylactic Co-Trimoxazole to help prevent a specific kind of pneumonia known as PCP. PCP is a type of fungus which can infect the lungs. It is also known as pneumocystis jiroveci. You may be offered prophylactic Co-Trimoxazole if your immune system has become weakened for any reason or are taking medications that stop it from working well.

When should I avoid Co-Trimoxazole?

You should not take Co-Trimoxazole if:

  • You are allergic to trimethoprim or sulfamethoxazole

  • You have been diagnosed with a rare genetic condition called acute porphyria

  • You have severe liver failure

  • You have severe kidney failure

You should not take methotrexate and Co-Trimoxazole together. Methotrexate is a drug prescribed by specialist doctors to treat certain cancers and rheumatoid conditions (such as rheumatoid arthritis and some autoimmune conditions). Taking methotrexate and Co- Trimoxazole together can be dangerous and you should not do so unless your hospital consultant has specifically advised it.

Other conditions can also impact on your ability to take the drug safely. You should tell your doctor if:

  • You have any medical conditions affecting your blood cells (you may be seeing a specialist doctor known as a haematologist for this condition)

  • You have previously had a deficiency of a type of vitamin known as folate

  • You have had high levels of a salt known as potassium in your blood

Drug Interactions

Like most medications, Co-Trimoxazole may interact with other drugs. These include:

  • Drugs that impact on how your body processes a type of vitamin known as folate. These drugs are often used to treat rheumatoid diseases (such as rheumatoid arthritis and certain autoimmune conditions), or specific types of cancer.

  • Blood thinning medications (anticoagulants) including warfarin. This is because the blood thinning effects of warfarin can be increased by Co-Trimoxazole.

Your doctor can advise whether it is safe for you to take the drug with any other medications you are on which may interact with Co-Trimoxazole.

Make sure you tell your doctor about all medications you are already taking. This includes any medications, vitamins, or supplements that you buy over the counter.

Pregnancy and Breast Feeding

You should not take Co-Trimoxazole if you are pregnant.

Co-Trimoxazole is generally safe to use if you are breast feeding. However, there are some situations where it may not be safe to use. For example, if your baby is jaundiced (yellowing of your baby’s skin or eyes).

If you are breast feeding, you should always check with your doctor or midwife before taking Co-Trimoxazole.

What dose should I take?

Your doctor will advise you how much Co-Trimoxazole to take. The dose prescribed will vary depending on the reason you have been prescribed the drug and whether you are taking treatment or prophylactic doses.

Treatment doses

To treat lung infections the standard dose is 960mg twice a day for between 5 and 7 days.

Treatment of PCP requires much bigger doses of antibiotic and the treatment is longer, often lasting up to 3 weeks. If you need treating for PCP your doctor will explain how many tablets you need to take and how often.

Prophylactic doses

Dosing regimens to prevent PCP infection vary depending on why you are at risk. Your consultant will be able to explain the specific regimen they use and what you will need to do. Sometimes you may not need to take the drug every day.

How should I take Co-Trimoxazole?

Co-Trimoxazole can be prescribed as:

  • Tablets

  • Liquid

  • An intravenous infusion - Co-Trimoxazole will be given directly into the veins and will usually be done in hospital

You should try to take Co-Trimoxazole with food and water. This will help to stop you feeling sick. You should also try to ensure you remain well hydrated. To do this you should try to drink approximately 8 glasses of water per day.

If you miss a dose of Co-Trimoxazole you should take the tablet as soon as you remember. If you don’t remember until your next dose is due, take that tablet as planned and continue until you finish the course. Do not take two doses at once.

If you take too much Co-Trimoxazole you should phone 111 urgently or go to the emergency department immediately for advice.

Can I drink alcohol?

Drinking alcohol whilst taking Co-Trimoxazole can occasionally make you feel:

  • Sick

  • Flushed

  • Vomit

  • Drowsy

  • Headache

  • Pain in your chest or abdomen (tummy)

This reaction is very rare. It is usually safe to drink alcohol in moderation whilst taking Co-Trimoxazole.

You should still follow the usual advice regarding alcohol consumption. This includes drinking no more than 14 units of alcohol per week. This should be spread out over several days. You should avoid binge drinking.

If you are feeling unwell, or are experiencing side effects, alcohol may worsen your symptoms as it can cause dehydration, nausea, and vomiting.

Will I get any side effects?

Like all medications, Co-Trimoxazole comes with possible risks and side effects. However, these will not affect everyone.

The potential risks and side effects need to be balanced with the potential benefits that Co- Trimoxazole has in treating serious infections.

Common side effects affecting 1in 10 people include:

  • Thrush

  • Rash

  • Nausea

  • Diarrhoea

More rarely people may experience:

  • Muscle aches

  • Tinnitus or ringing in the ears

  • Changes to appetite

If you notice any of the above side effects, you should stop the treatment and speak with the doctor who prescribed your medication as soon as possible.

Co-Trimoxazole can also sometimes make your skin sensitive to sunlight. Remember to:

  • Wear sunscreen

  • Avoid activities like sunbathing

Some rare side effects of Co-trimoxazole can be very serious and, in some instances, can be life threatening. These side effects can affect different parts of the body or cause a very severe allergic reaction called anaphylaxis.

These are documented in a little more detail below. If any of the symptoms documented below occur, you need to stop the drug and seek immediate medical help.


This is a severe allergic reaction and usually occurs very rapidly after taking the drug. If you notice:

  • Swelling in your face or lips

  • Breathing difficulties

  • Fainting

  • A severe rash

You should seek medical help immediately and stop the drug.

Bruising or easy bleeding

Sometimes Co-trimoxazole can affect the bone marrow and stop it working efficiently so your blood cannot clot properly and you cannot fight infection. You might notice:

  • A sore throat

  • Easy bruising or bleeding

  • A fever

  • Feeling very tired

If you notice any of these symptoms you will need urgent blood tests and review in hospital as these problems can be very serious.

High potassium levels

Potassium is a salt in the blood and high levels of this can cause problems such as:

  • Palpitations (feeling like your heart is beating irregularly)

  • Chest pain

  • Tingling in your hands or feet

There are lots of other causes for these symptoms but if they happen whilst taking Co-Trimoxazole you need to have an urgent blood test to check your kidney function and make sure your potassium level remains safe.

Liver failure

Liver failure is a rare but serious complication of co- trimoxazole use which can be fatal. Signs of liver failure include:

  • Itching

  • Pale poo

  • Yellowing of your eyes or skin (jaundice)

  • Pain or swelling of your abdomen (tummy)

If any of these occur, you must seek urgent medical help

You can find more information about possible side effects in the drug information leaflet that came with your prescription. If you are worried about side effects or have symptoms you are worried about you should speak to your doctor.

What if I cannot take Co-trimoxazole?

If you cannot tolerate these tablets there will be other antibiotic options available for you. You can discuss these with your doctor.

Will I require any monitoring?

If you are taking Co-Trimoxazole for a prolonged period, you will need to have regular blood tests. These blood tests are done to check that your kidneys (U+Es check)and liver (LFTs check) are working well. They will also check the levels of different cells in your blood (FBC check) to make sure your red blood cell level is not too low and that you are not at an increased risk of infection. Your doctor will advise you how frequently you will need monitoring.

The dates for your monitoring can be recorded in a table on the inside of the back cover of this booklet.

Additional blood tests can be done at any time in between if new problems develop.

Who should I contact if I have any questions?

If you have any other questions of queries, please contact the medical team who have prescribed you Co- Trimoxazole. The team providing your care will fill in their contact details in the space below.

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