If you’re anything like me, we try to avoid using the infamous SMZ-TMP at all costs. I’m here today to repent for my sins against sulfonamide antibiotics, and to say that Bactrim is not as bad as it was made out to be.
INTERACTIONS:
An argument can be made that the number one reason prescribers avoid Bactrim is due to perceived drug/drug interactions. Arguably the most notable interaction between Bactrim and another agent is between Bactrim and warfarin, whereby Bactrim can increase the anticoagulant effects of warfarin, warranting a 10-20% dose reduction. With the decline in popularity in warfarin, this interaction is becoming less and less common. Absolutely, you should avoid Bactrim for your warfarin patients if possible. However, if it’s generally unavoidable, such as when treating a resistant Stenotrophomonus spp. infection, it’s a manageable interaction.
The next interaction to be aware of is regarding agents that already increase the risk for hyperkalemia: ACE-inhibitors, ARBs, and spironolactone. The trimethoprim component of Bactrim actually inhibits sodium channels in the distal nephron, which leads to inhibition of potassium secretion into the urine. However, if you have a normal baseline potassium when you see your patient and are thinking of discharging them on a short course of Bactrim, this interaction is something to be aware of, but not something I would say “let’s pick literally anything else” over.
These are the only truly significant drug interactions that I look for on every patient being prescribed Bactrim. Of course there are other individuals on medications that have interactions to some extent, but the above interactions I would say are 90% of all Bactrim interactions in the real world.
ADVERSE EFFECTS:
HYPERKALEMIA – When combined with other agents that increase potassium.
DRUG-INDUCED LIVER INJURY – Not common in my experience but avoid use in patients with liver impairment if possible. May also cause transient tramsaminase elevations.
BONE MARROW SUPPRESSION – Immune related process. Pancytopenias including agranulocytosis, leukopenia, thrombocytopenia, and hemolytic anemia. Possibly related to DRESS (drug reaction with eosinophilia and systemic symptoms).
OTHER – Of course there are other adverse effects regarding Bactrim, namely Renal Failure and Rash including SJS, but I feel the above adverse effects are the most notable and serious.
THINGS TO KNOW:
MECHANISM: Bactrim is a folate antagonist, which interrupts bacterial folate synthesis, leading to inhibition of DNA synthesis.
SPECTRUM/USES: Skin/Soft tissue infections (including MRSA), urinary tract infections, diverticulitis/other uncomplicated intra-abdominal infections (with metronidazole), infectious diarrheas (salmonella/shigella), and infections with H. influenzae, Stenotrophomonus maltophilia, Listeria spp., and Pneumocystis jirovecii.
FORMULATION: Tablets are in a 1:5 ratio of trimethoprim:sulfamethoxazole – 80mg-400mg or 160-800mg tablets. IV formulation contains propylene glycol, which CONTRAINDICATES IV BACTRIM WHEN USED WITH METRONIDAZOLE due to a likely disulfiram reaction.
ALLERGIES: Cross-reactive with allergies to other sulfa-based products, avoid if sulfa allergy is noted.
SUMMARY:
All in all, Bactrim does have some intricacies to prescribing. However, it’s a viable and likely safe option for patients needing treatment for uncomplicated skin/soft tissue infections, urinary tract infections, and uncomplicated intra-abdominal infections (please add Flagyl for anaerobic coverage on those abdominal infections though). If you’re aware of these risks and the patient is confirmed to not be on warfarin, there’s a 95% chance that Bactrim will be just fine. When you put into perspective that 10% of Americans report some sort of allergy to penicillin, those odds look pretty darn good.