(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 might not be strong enough to do so. 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. Similarly, trimethoprim-sulfa can penetrate "sequestered" sites of the body where there is a natural barrier separating certain tissues from the rest. 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, however, not generally effective against Pseudomonas auruginosa, which is particularly resistant where ever it emerges.
Trimethoprim sulfa is generally prescribed for once or twice a day use. It may be given with or without food and should be stored at room temperature. 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:
In general, drug side effects are classified as either common or serious (with rare drugs having side effects that are both common and serious). As with most drugs, trimethoprim sulfa has common side effects and rare serious side effects.
COMMON SIDE EFFECTS
Upset stomach symptoms of vomiting/nausea, diarrhea, or appetite loss can be seen with this medication. The oral suspension is famous for its awful taste and may elicit drooling as the pet tries to get the taste out of his or her mouth. These side effects are inconvenient and may even necessitate changing to another medication but are not regarded as serious.
Inability to produce adequate tears ("dry eye")
SERIOUS SIDE EFFECTS
Watch for facial swelling, difficulty/painful movement, listlessness, cloudy eyes.
Watch for extremely itchy skin, sores.
Watch for yellow pigmentations of the skin and eyes.
Watch for bruising, bleeding, weakness, paleness of the gums.
Watch for yellowing of the eyes and skin, weakness.
Sulfa bladder stones
Watch for bloody urine, straining to urinate.
INTERACTIONS WITH OTHER DRUGS
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: 6/17/2020