How Long Does Bactrim Stay In Your System?

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Jessica Guht

Jessica is a medical writer with an unquenched thirst to discover something new. She believes that medical content should be accessible to everyone and strives to write content that every single person can understand. When Jessica isn’t writing, she can usually be found reading a book with a dog cuddled in her lap. Jessica has a Masters of Engineering degree in Biomedical Engineering.

Medically reviewed by

Amy Dougherty, FNP-BC, AGAC

Amy is a Board Certified Family Health Nurse Practitioner (FNP) with over 15 years of experience working in Hospital Medicine, Urgent Care and Primary Care practices. Amy graduated Thomas Jefferson University with high distinction earning a Bachelor of Science in Nursing in 2008, a Master of Science in Nursing in 2010 and a Post Master's Certificate in Adult Gerontology Acute Care (AGAC) in 2014. She was recognized by the Elite American Nurses Association in 2013 for her dedication, achievements and leadership in the field Nursing. She served as a clinical preceptor for a number of Nurse Practitioner students and enjoys teaching the bright minds of future NPs.

Bactrim is a combination antibiotic consisting of trimethoprim and sulfamethoxazole. It helps fight bacterial infections but can also cause some unpleasant side effects.

Depending on how severe these side effects may be, you may choose to discontinue this medication after talking to your doctor, but your side effects might not go away until Bactrim is out of your system.

As for how long Bactrim stays in your system, it remains for around five days after your last dose, although various factors can modify this length. Still, if your symptoms do not improve within a few days after stopping Bactrim, reach out to a doctor. 

Table of Contents

• What Is Bactrim?
• How Long Does Bactrim Stay In Your System?
• How Long Do the Side Effects of Bactrim Last After Your Last Dose?
• When to See a Doctor?
• Key Takeaways

What Is Bactrim?

Bactrim is the brand name for the combination of sulfamethoxazole/trimethoprim, which are both antibiotics responsible for treating different types of bacterial infections. These antibiotics work together to block two steps in the production of nucleic acids and proteins bacteria need to survive. 

Bactrim is usually taken in doses every twelve hours, and the length of treatment varies based on your infection and its severity, but it may span anywhere from 3 to 14 days. It is also available in double strength tablets as Bactrim DS.

Uses

Bactrim is used to treat bacterial infections such as:

• urinary tract infections (UTIs)
• ear infections
• traveler’s diarrhea
• pneumocystis jiroveci pneumonia
• shigellosis

Of note, some of the above illnesses can result from bacterial or viral infections. As an antibiotic, Bactrim will only be effective if the cause of the infection is bacterial. 

Side Effects

Like most antibiotics, some of the most common side effects of Bactrim revolve around stomach upset, with diarrhea, in particular, a common side effect, along with nausea, vomiting, or loss of appetite. 

Additionally, Bactrim can make your skin more sensitive to sunlight, which can cause you to sunburn more easily. As such, wearing protective clothing when outside, using sunscreen, and avoiding sunlight or tanning beds when possible is recommended. 

Some people may be allergic to Bactrim, with signs of this including coughing, hives, shortness of breath, chest pain, or swelling in the face or throat. If you experience any of these symptoms, seek emergency medical attention. f

Bactrim may also cause a severe skin reaction with symptoms that include sore throat, fever, skin pain, burning eyes, or a purple or red rash with peeling skin and blisters. Again, if you experience any of these symptoms, seek medical attention. 

How Long Does Bactrim Stay In Your System?

How long Bactrim stays in your system depends on its half-life. Because Bactrim is composed of two different drugs, we need to look at the half-life for each to see the maximum amount of time the drug will remain in your body. 

The half-life of sulfamethoxazole is 10 hours, and the half-life of trimethoprim can range from 8-10 hours. Even though Bactrim is a combination drug, these half-lives don’t add onto each other as both drugs begin breaking down at the same time. So, the maximum half-life would remain 10 hours, and wouldn’t add up to 20 hours.

Now that we know the half-life of Bactrim, we can estimate how long it will stay in your system. Since it generally takes a drug 5-6 half-lives to be entirely removed from the body, it can take 50-60 hours before Bactrim is completely out of your body after your last dose or around 5 days. 

While this is the average amount of time that Bactrim will stay in your system, certain factors can influence it. For example, those who are older often keep drugs in their system for longer. Diet and weight can also influence how long a medicine remains in your system.

Along these same lines, if you have certain medical conditions that influence how well your body can clear out medication, such as kidney disease, it will likely take longer until the drug is entirely out of your system. 

How Long Do the Side Effects of Bactrim Last After Your Last Dose?

Knowing that Bactrim can stay in your system for around 5 days (sometimes more, sometimes less), it’s natural to wonder just how long you may experience Bactrim’s side effects even after you stop taking the medication or have completed your course of treatment. 

In general, Bactrim’s side effects should improve within a few days or up to a week after you stop taking it. 

However, many things can influence how long you may experience side effects, such as your overall health, the side effects you are experiencing, and the dosage of the medication that you are taking. 

Looking at each side effect, those such as nausea and vomiting generally go away within a few days, and taking Bactrim with food can further alleviate them. Once you stop taking Bactrim, these side effects usually go away quickly. 

However, other side effects may last longer. For example, those who develop a skin rash from Bactrim can see variations in how long it lasts based on the rash’s severity and cause. 

FAQ

When to See a Doctor?

If you are taking Bactrim and notice the development of any unpleasant side effects, especially any that make you want to discontinue the medication, reach out to a healthcare provider first. They may be able to offer advice to lessen the side effects, such as taking Bactrim with food to relieve nausea or staying hydrated and avoiding spicy and greasy food to improve diarrhea. 

However, if you notice any severe skin reactions, talk to a doctor immediately:

• peeling
• blistering
• skin rash
• itching
• loosening skin
• ulcers
• sores
• red skin lesions
• white spots on your lips or mouth

There is also a potential risk of allergic reaction with Bactrim, with symptoms such as difficulty breathing, itching, swelling of the throat, tongue, or mouth, and chest pain. If you experience any of these symptoms, call a doctor or visit the emergency room. 

Bactrim may also cause liver problems in some people. Again, seek immediate medical attention if you have any of the following symptoms:

• lightening stool
• darkening urine
• jaundice (yellowing of the skin or eyes)
• stomach pain

Key Takeaways

Bactrim is a combination antibiotic that consists of sulfamethoxazole/trimethoprim. These two antibiotics work together to treat a range of bacterial infections. Unfortunately, like most antibiotics, Bactrim can have some side effects that generally last as long as you take it. For example, it can make your skin more sensitive to sunlight, so staying out of the sun is recommended as long as Bactrim is in your system.

This raises questions regarding how long Bactrim remains in your system, but generally, it stays in your system for around 5 days after your last dose. Knowing this information is essential for monitoring side effects and seeking medical attention if they do not go away after a few days. 

Sources:

• Sulfamethoxazole / Trimethoprim – Tablet, tablet (double strength) | NIH. (2023). https://clinicalinfo.hiv.gov/en/drugs/sulfamethoxazole-trimethoprim/tablet-tablet-double-strength
• Kemnic TR, Coleman M. Trimethoprim Sulfamethoxazole. [Updated 2022 Nov 28]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2023 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK513232/
• Bactrim. (2013). https://www.accessdata.fda.gov/drugsatfda_docs/label/2013/017377s068s073lbl.pdf 
• Hagihara, M., Kato, H., Uchida, S., Yamashita, R., Tanaka, S., Sakanashi, D., Shiota, A., Asai, N., Koizumi, Y., Suematsu, H., Yamagishi, Y., Namiki, N., & Mikamo, H. (2019). The First Report on Pharmacokinetic/Pharmacodynamic Study of Trimethoprim/Sulfamethoxazole against Staphylococcus aureus with a Neutropenic Murine Thigh Infection Model. Chemotherapy, 64(5-6), 224–232. https://doi.org/10.1159/000507540 
• Haseeb, A., Abourehab, M., Almalki, W., Almontashri, A., Bajawi, S., & Aljoaid, A. et al. (2022). Trimethoprim-Sulfamethoxazole (Bactrim) Dose Optimization in Pneumocystis jirovecii Pneumonia (PCP) Management: A Systematic Review. International Journal Of Environmental Research And Public Health, 19(5), 2833. doi: https://www.doi.org/10.3390/ijerph19052833 
• Kato, H., Hagihara, M., Asai, N., Umemura, T., Shibata, Y., & Hirai, J. et al. (2022). Efficacy of Trimethoprim–Sulfamethoxazole in Combination with an Echinocandin as a First-Line Treatment Option for Pneumocystis Pneumonia: A Systematic Review and Meta-Analysis. Antibiotics, 11(6), 719. doi: https://www.doi.org/10.3390/antibiotics11060719