Reviewed by Chris Woods, MSN, APRN, FNP-C — Licensed Nurse Practitioner. This guide is for educational purposes and is not a substitute for medical advice. Always follow your provider's instructions.
What Is Bactrim?
Bactrim (brand names: Bactrim, Bactrim DS, Septra) is a combination antibiotic containing sulfamethoxazole and trimethoprim in equal parts. It's a sulfonamide-type antibiotic commonly prescribed to treat urinary tract infections and other bacterial infections.
The Two-Antibiotic Advantage: Trimethoprim and sulfamethoxazole work at different steps in the bacterial cell's process of making folic acid — one of the chemicals bacteria need to survive and multiply. By attacking bacteria from two angles at once, Bactrim is more powerful than using just one antibiotic.
Bactrim comes in two common strengths:
- Bactrim (regular strength): 400mg sulfamethoxazole / 80mg trimethoprim per tablet
- Bactrim DS (double strength): 800mg sulfamethoxazole / 160mg trimethoprim per tablet — this is what's usually prescribed
What Does Bactrim Treat?
Bactrim is prescribed for bacterial infections, most commonly urinary tract infections (UTIs). It's also used for other infections in certain situations.
Important: Bactrim should NOT be used if you have a sulfa drug allergy. This is very serious. Tell your provider immediately if you've ever reacted to sulfa medications.
How to Take Bactrim
Taking Bactrim correctly is important for the medication to work well and to reduce your risk of side effects.
Dosing
| Condition |
Dose |
Duration |
| Uncomplicated UTI |
1 Bactrim DS tablet twice daily |
3-5 days |
| Recurrent UTI Prevention |
1 Bactrim DS tablet at bedtime |
As directed (may be weeks to months) |
Instructions
- Take with food or without: Bactrim can be taken with or without food. If you get an upset stomach, take it with food or milk.
- Space doses evenly: Take your doses about 12 hours apart (like 8am and 8pm). This keeps the medication at a steady level in your body.
- Finish the full course: Even if you feel better, take all of your Bactrim as prescribed. Stopping early can cause the infection to come back stronger and can lead to antibiotic resistance.
- Drink plenty of water: Drink at least 8-10 glasses (2-3 liters) of water per day while taking Bactrim. Staying hydrated helps your kidneys handle the medication and reduces side effects.
Hydration is Key: The more water you drink, the better Bactrim works and the fewer side effects you'll have. Don't hold back on water intake — your kidneys need it to filter this medication safely.
Side Effects & What to Watch For
Most side effects from Bactrim are mild and go away after you finish the medication. But some require immediate medical attention.
Common Side Effects
- Nausea or vomiting
- Loss of appetite
- Rash or itching (mild)
- Dizziness
- Headache
- Sensitivity to sunlight
- Diarrhea
Serious Side Effects — CALL IMMEDIATELY
- Severe allergic reaction (swelling of face, throat, difficulty breathing)
- Severe rash or skin blistering (Stevens-Johnson Syndrome)
- Mouth sores or severe sore throat
- Unusual bruising or bleeding
- Severe abdominal pain or bloody stools
- Dark urine or yellowing of skin/eyes
- High fever with rash
- Severe joint or muscle pain
Red Flag Alert: If you develop a rash while taking Bactrim, watch it closely. Most rashes are mild, but some can be serious. If the rash is spreading, has blisters, involves your mouth or eyes, or comes with fever, stop the medication and call your provider right away.
Important Warnings & Drug Interactions
Before taking Bactrim, your provider needs to know about certain conditions and medications you're taking.
Sulfa Allergy
This is critical: If you are allergic to sulfa drugs or sulfonamides, do NOT take Bactrim. Tell your provider right away. There are other antibiotic options available.
Sun Sensitivity
Bactrim makes your skin very sensitive to the sun. While taking it:
- Wear broad-spectrum sunscreen (SPF 30+) every single day
- Wear protective clothing (long sleeves, hat) when you go outside
- Limit direct sun exposure, especially midday
- Avoid tanning beds
Important Medical Conditions
Tell your provider if you have:
- Kidney disease or reduced kidney function
- Liver disease
- Folate deficiency or anemia
- G6PD deficiency (a blood condition)
- Pregnancy (especially 1st trimester or near delivery)
Major Drug Interactions
Tell your provider about these medications before taking Bactrim:
- Warfarin (blood thinner): Bactrim can increase warfarin's effects and raise bleeding risk
- Methotrexate: Can increase methotrexate levels and toxicity
- ACE inhibitors (for blood pressure): May increase potassium levels — monitor closely
- Potassium-sparing diuretics: Can cause dangerously high potassium levels
- Certain diabetes medications: May need dose adjustment
Frequently Asked Questions
What's a sulfa allergy and how do I know if I have one?
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A sulfa allergy is a reaction to sulfonamide drugs (medications containing sulfur). Signs of a sulfa allergy include rash, itching, fever, swollen lymph nodes, or joint pain. If you've had a reaction to sulfa drugs before — like Bactrim, Gantrisin, or other sulfa medications — you likely have a sulfa allergy. Tell your provider immediately. There are other antibiotics that can treat your infection safely.
Is Bactrim the same as Bactrim DS?
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No, they're different strengths. Bactrim DS (double strength) contains 800mg sulfamethoxazole and 160mg trimethoprim. Regular Bactrim has 400mg and 80mg of those same drugs. Bactrim DS is prescribed more commonly because you take fewer tablets. Your provider will choose the right strength for your specific infection.
How much water should I drink while taking Bactrim?
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Drink plenty of water — at least 8-10 glasses (2-3 liters) per day while taking Bactrim. Staying hydrated helps protect your kidneys and reduces your risk of side effects. More water is better. If you drink a lot of water, you'll feel better and have fewer problems from the medication.
Can I go in the sun while taking Bactrim?
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Bactrim makes your skin much more sensitive to the sun. You can still go outside, but you MUST protect yourself. Wear broad-spectrum sunscreen (SPF 30+), protective clothing, and limit direct sun exposure while taking it. Sunburns can happen much faster than normal. After you finish taking Bactrim, your sun sensitivity goes away.
How quickly will my UTI symptoms improve?
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Most people start feeling better within 24-48 hours of starting Bactrim. Burning with urination often improves first. But don't stop taking it just because you feel better. Finish your entire course — usually 3-5 days for uncomplicated UTI — to make sure the infection is completely gone. Stopping early can cause the infection to come back stronger.
Sources & Citations
- U.S. Food and Drug Administration (FDA). DailyMed: Sulfamethoxazole and Trimethoprim Tablets. Available at https://dailymed.nlm.nih.gov
- Infectious Diseases Society of America (IDSA). International Clinical Practice Guidelines for the Treatment of Uncomplicated Cystitis in Women. Available at https://www.idsociety.org
- UpToDate: Acute Uncomplicated Cystitis in Women. Available at https://www.uptodate.com
- American Academy of Family Physicians (AAFP). Recurrent Urinary Tract Infections in Women. Available at https://www.aafp.org
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