Bactrim Side Effects: Generic name trimethoprim/sulfamethoxazole.  Other brand names are Sulfatrim and Septra.  Bactrim is an antibiotic in one of the oldest classes of antibiotics commonly called sulfa drugs. Among the most common Bactrim side effects are allergic reactions including rashes of varying severity as well as other allergic reactions.   Bactrim is available in 80/400 mg dosage, meaning 80 mg of trimethoprim and 400 mg of sulfamethoxazole. These are used in combination because the combination is much more effective as an antibiotic than either alone.  Bactrim DS is simply a double strength product of 160/800 strength.  The generic product is also available as pediatric syrup at the strength of 40/200 per 5ml.  The common adult dose for most indications is one Bactrim DS twice daily.  Bactrim is used in treatment of many bacterial infections, especially urinary tract infections and more recently for MRSA (methicillin resistant staph aureus) infections.  Less common indications are treatment and prevention of pneumocystis carinii pneumonia in HIV and other immunosuppressed patients and for treatment of salmonellosis and typhoid fever.

Common Bactrim Side Effects:  Some of the most common Bactrim side effects are gastrointestinal problems including nausea, vomiting, loss of appetite, and gastrointestinal upset of other types.  Other Bactrim side effects that seem more related to the central nervous system include photosensitivity, loss of energy, headache and dizziness.  The most important and common Bactrim side effects are allergic reactions.  These are really very common, and so many people are allergic to the sulfa class of medications that it often limits the use of Bactrim.  Most of the allergic reactions are mild to moderate rashes and hives.   More serious allergic reactions are discussed below.

Serious Bactrim Side Effects:  The most common serious Bactrim side effects are allergic reactions.  Although most of the skin rashes from Bactrim allergies are mild to moderate in severity both Stevens-Johnson Syndrome and toxic epidermal necrolysis are common enough to keep in mind. Hematologic serious Bactrim side effects include agranulocytosis, aplastic anemia, myelosuppression and serious blood dyscrasias.  Other serious internal organ Bactrim side effects include fulminant hepatic necrosis, hepatitis and hepatotoxicity, interstitial nephritis, pulmonary infiltrates, goiter and aseptic meningitis.  Additional serious Bactrim side effects are anaphylactoid reactions, kernicterus (in neonates), methemogloginemia, hyperkalemia and systemic lupus erythematosis.   Pseudomembranous colitis is a well-documented Bactrim side effect, much as it is with Doxycycline side effects and Levaquin side effects.

Bactrim Drug Interactions:  Bactrim cannot be used with BCG, dofetilide and methenamine.  Bactrim affects the metabolism of warfarin and special care needs to be taken or Bactrim use avoided in patients on warfarin.  Many drugs are affects by Bactrim, but of the more commonly used and potentially serious drug interactions amiodarone and methotrexate warrant mention here.  See the prescriber’s extensive list of drug interactions for a complete list.  Among these of note is digoxin, several HIV drugs and some herbal supplements like St. John’s wort and black cohosh.

Pregnancy and Lactation:  Bactrim is pregnancy category C, but this varies with trimester and you should discuss use during pregnancy with your physician.  Use in lactation varies with several factors including the gestational age of the infant, and needs to be discussed with your physician.

Unusual Side Effects:  The high incidence and variety of allergic reactions, especially the skin rashes are unusual in that they affect such a high percentage of patients.

Special Bactrim Side Effects considerations:  Something to keep in mind is that numerous commonly used medications, especially the thiazide diuretics, are listed as cross reacting with the sulfa medications for allergic reactions.  The thing to keep in mind about this is that most patients with Bactrim allergy can use these drugs, because the sulfa moiety is chemically different and only about 3% of patients with an allergy to sulfa drugs also develop an allergic reaction to thiazide diuretics.

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