(for veterinary information only)
BRAND NAMES: TRIBRISSEN, SEPTRA, BACTRIM, COTRIM, SULFATRIM
Ever since the development of penicillin, there has been a drive to make antibiotics better: better ability to penetrate into infected tissue, better efficacy against a broader spectrum of bacterial organisms, less potential to harm host tissues. The combination of trimethoprim and sulfa antibiotics have created a very unique method to combat bacteria: the “sequential blockade” which we will describe below.
An essential nutrient used in the synthesis of many important biochemicals is Folic acid. Folic acid is made from para-amino benzoic acid (PABA) through a step by step process involving two enzymes. The sulfa drug inhibits the first enzyme and trimethoprim inhibits the second enzyme. This double inhibition is called the “sequential blockade” and produces death of the bacterium whereas either antibiotic alone could only have inhibited bacterial reproduction. Mammal enzymes are far less sensitive to the blockade than bacterial enzymes but what really protects the infected host from the blockade is the simple fact that mammals do not have to manufacture their own folic acid; they can eat it in their diet.
HOW THIS MEDICATION IS USED
There are several special advantages to using trimethoprim sulfa. First, this medication has the special advantage of being able to penetrate into exudates and infected tissues that usually stop other antibiotics at their surface. This means trimethoprim sulfa can enter not only abscessed tissue but can penetrate the prostate gland, the blood brain barrier, and eye and treat infections in these locations.
Trimethoprim sulfa is a broad spectrum antibiotic with excellent activity against most gram negative organisms and against Staphylococci in the skin. This makes trimethoprim sulfa a good choice for skin infections or as a general antibiotic when the actual identity of the infecting organism is not known. Trimethoprim sulfa is not generally effective against Pseudomonas auruginosa.
Another advantage of trimethoprim sulfa is that it has minimal effect on the normal flora of the GI tract of the patient. This means less potential for antibiotic induced diarrheas and less resistant bacteria in the home.
Trimethoprim sulfa is given twice a day. It is of relatively low cost compared to other antibiotics which makes it a popular choice.
Infections for which trimethoprim sulfa are especially helpful are:
Trimethoprim-sulfa reactions are uncommon but a number of reactions have been well-described so it is important to be familiar with them particularly if your pet must take Trimethoprim-sulfa for more than three weeks. Reactions have potential to be serious and they occur unpredictably (with the possible exception of reactions in the Doberman pinscher, see below.) The following syndromes have been associated with trimethoprim-sulfa use:
Inability to produce adequate tears ("dry eye")
Sulfa bladder stones
INTERACTIONS WITH OTHER DRUGS
Trimethoprim sulfa will interfere with thyroid function testing. It is not known precisely how long trimethoprim sulfa should be discontinued in order to get a valid thyroid reading.
The following drugs may be enhanced by trimethoprim sulfa use: phenylbutazone (an NSAID), thiazide diuretics, aspirin, and methotrexate (a cancer medication).
Antacids may interfere with the efficacy of Trimethoprim sulfa.
Trimethoprim sulfa should not be used with cyclosporine (used for airborne allergies, perianal fistulas, and immune suppression after organ transplant). This combination increases toxicity of the cyclosporine and reduces its beneficial effects.
Concurrent use of thiazide diuretics with trimethoprim-sulfa can lead to a significant drop in platelets.
Tricyclic antidepressants may not be as effective if used in conjunction with trimethoprim-sulfa.
CONCERNS AND CAUTIONS
It is important to become familiar with the above described idiosynchratic reactions.
Signs of overdose include nausea and diarrhea, confusion and depression, bone marrow disease, and facial swelling.
Page last updated: 3/27/2016