HEADS UP!! Bactrim Dosing

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Post   » Sun Sep 18, 2016 7:46 pm

Recently one of our members (Delaine?) was given Bactrim (sulfa trimeth) at the vet, at a concentration nearly nine times the standard "usual", yet the dose was given as per normal -- resulting in a nine-fold overdose had it not been caught.

The same thing happened to me this past Thursday. It took two phone calls to the vet to straighten it out.

The standard "pink stuff" that vets and caretakers have used for years is 240 mg/5 mL, or 48 mg/mL. This is the standard pediatric suspension, and is what is referenced in our Medical Guide here.

The "new" stuff is 400 mg/mL. It can of course be used but the dosing volume has to be decreased accordingly! The stuff I got was a pale goldish-beige, which is what tipped me off that there might be a problem.


I suspect the vets are so accustomed to using this med that they prescribe as they have for years, while some drug manufacturer is pushing a new formula to clinic purchasing personnel. Make sure everyone is on the same page before you leave the vet if you are given Bactrim for your pig!

(Lynx -- I have taken the liberty of making this a sticky. If that's inappropriate, I apologize, and please adjust as you see fit.)

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Post   » Sun Sep 18, 2016 10:11 pm

I will add a warning on the antibiotic page tomorrow.

And got the T-shirt

Post   » Sun Sep 18, 2016 10:17 pm

Thanks for adding that, Talishan. I've posted a warning at GPC also.

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Post   » Mon Sep 19, 2016 1:09 am

Thank you both.

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Post   » Sun Sep 25, 2016 1:29 pm

I am looking for something solid online describing precisely this drug. I can't find it.

Did find this but the oral is still the same-old same-old pink stuff. Perhaps one of the other concentrations delivered parenteraly? I had to look it up as it just means not administered via the mouth and alimentary canal. "Taken into the body or administered in a manner other than through the digestive tract, as by intravenous or intramuscular injection."


Dosage: The 1:5 ratio (TMP:SMX) remains constant in all dosage forms

- Tablets Single strength (SS: 80/mg/400mg TMP/SMX)
. . . . . . . . Double strength (DS: 160mg/800mg TMP/SMX)

- Liquid (suspension) 40mg / 200mg TMP/SMX per 5ml

Parenteral: Vial
- 5ml: Single strength (80mg/400mg TMP/SMX)
- 10ml: Double strength (160mg/800mg TMP/SMX)
- 30ml: Six times strength (480mg/2400mg TMP/SMX)

I would like to find the higher concentration product sold online so I can add better information.

And got the T-shirt

Post   » Sun Sep 25, 2016 1:50 pm

It could have been compounded.

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Post   » Sun Sep 25, 2016 2:19 pm

Talishan referred to the "new stuff" being a pale goldish beige. It sounds like another suspension is available at a different concentration. This is what I'm trying to track down.

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Post   » Tue Oct 18, 2016 10:01 am

I hope your little one was able to recover from the dosing error, Talishan. Thinking of you...


Post   » Sun Oct 28, 2018 9:09 pm

Hi, gang.

I'm a new member of the forum. Besides being a "dad" of three guinea pigs (Buttons, Teddy, and Albus....he prefers Albie.....I'll share them in another thread later), I'm also a pediatrician. I have a lot of experience prescribing trimethoprim/sulfamethoxazole (TMP/Sulfa), so I guess I'll add a few comments to this topic, if that's ok.

The most common usages I find myself writing for it for kids is for coverage of UTI's (sometimes changed to another antibiotic pending culture results), and for superficial skin infections when MRSA is being considered.

At least here in the US, the drug comes in various brand names, including Bactrim, Sepra, and Sulfatrim, but the generic is also available.

I was fortunate to come across this forum a couple years ago when our oldest pig, Buttons, was sick with a suspected respiratory infection. The advice on the forum was very valuable, and after learning the recommended guinea pig bactrim dosage, I was able to weigh him, figure his dose, and he got all better. Our regular vet was away on vacation at the time, so I was forced to act outside of my typical clientele. :-)

A few points I will add then:

With kids, similar to guinea pigs apparently, medications are calculated on a mg/kg basis. There are also typically various concentrations of oral suspensions available, and then one has to know how often to give the medicine.

Opportunities for errors exist, therefore, on multiple fronts. I'll go over a few of these so people can watch out for them.

First, as highlighted in this thread topic, is watching your suspension concentration. The only concentration I have ever written for is the TMP 40mg/Sulfa 200 mg per 5 ML suspension. This is the concentration most readily available, and my advise I guess would be for folks to stick with this.

Second, is understanding and being careful about the mg/kg dosing. If I were to say to give a pig "X" amount of mg/kg of the medicine, am I referring to the TMP component, the Sulfa component, or both? This has to be made clear. I was always taught to calculate Bactrim on the TMP component, which is what I do. You can do it, however, any of the three ways you want, but the critical issue is you have to know which of the three ways are being used when a "mg/kg" dose is recommended. It appears that on the antibiotic page of this forum, the method being used to base the calculation is the SUM of both components (the TMP and the Sulfa). I've never seen it dosed this way, so I took pause at it, but the math will work out as long as you pay heed to your numbers.

Finally, when a "mg/kg" dose is advised, does that refer to mg/kg/day, or mg/kg/dose? With Bactrim being dosed twice daily, this opens up the possibility of a 2x dosing error. The medication page on the forum does a good job of explaining that guinea pigs should receive 30 mg/kg/dose (given twice daily).

Hope that helps, and as mentioned, I look forward to later sharing with you all the adventures of our pigs!

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Post   » Sun Oct 28, 2018 10:42 pm

Thanks for the comments. Bactrim is unusual, with the two components.

One reference I have used over the years is James Carpenter's Exotic Animal Formulary, which in the rodent section lists:
Trimethoprim/sulfa 30 mg/kg PO, SC, IM q12h for all rodents, with a comment that tissue necrosis may occur when given SC (so we don't recommend it).
I don't recall where I learned (hopefully correctly) that the 30 mg is the compounded (added) amount but that seems to be the dose and it is written as 30mg/kg in vet books with no further clarification, so I think this may be a veterinary thing.

I did a search to see what I could find and ran across this sentence on page 856 of Veterinary Pharmacology and Therapeutics (a contemporary book):
"In dogs, pharmacokinetic studies have reported that 30 mg/kg (25 sulfadiazine + 5 trimethoprim) adminstered once daily should be adequate for most infections caused by susceptible organisms..."

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