For veterinary professionals
October 2026
You remove a bladder stone from a guinea pig, the family does everything you ask, and a few months later the radiograph shows another one. Or you flush a sludgy rabbit, send home fluids, and the bladder fills back up with grit as soon as the fluids stop. These are things we encounter in practice all the time, and they wear on everyone involved.
Dr. Thomas Donnelly and I have been adding two old, inexpensive drugs, hydrochlorothiazide (HCTZ) and potassium citrate, to the usual diet and husbandry changes for these patients. This isn't a protocol post. It's an explanation of why the combination makes physiologic sense, where it fits, and what to watch, so you can decide whether it's an option worth considering for your own stone formers.
Guinea pigs form mostly calcium carbonate uroliths. The likely contributors are a mix of mineral imbalance, high-calcium hays and vegetables, low water intake, genetics and underlying urinary disease such as cystitis or chronic kidney disease. The old teaching that this is mainly a disease of older sows is softening. Males and females now appear to be affected similarly, and stones turn up at any age. Recurrence after cystotomy is common, often within months.
Rabbits have an unusual calcium metabolism. They absorb dietary calcium largely passively, independent of vitamin D, and excrete the excess in urine. Feed alfalfa and urinary calcium climbs. Once the urine is supersaturated, sludge settles. Add obesity, osteoarthritis, pododermatitis, a small enclosure and a rabbit that can't posture comfortably to empty its bladder, and the sludge has nowhere to go.
Subcutaneous fluids help in the short term. In our experience, though, sludge returns once fluids stop, even when diet and exercise improve. We suspect that in moderate to severe cases the detrusor has been stretched and some urinary retention persists. Months of home fluids can also strain the human-animal bond, although good technician coaching helps a lot.
HCTZ inhibits the sodium-chloride cotransporter in the distal convoluted tubule. The result is diuresis together with increased calcium reabsorption, so less calcium reaches the urine. It is potassium-losing, which is one reason to pair it with potassium citrate.
Potassium citrate is a common point of confusion. In carnivores and people it's used as a urinary alkalinizer, but rabbit and guinea pig urine is already alkaline, and that isn't its job here. Urinary citrate complexes calcium into a water-soluble salt and inhibits crystal nucleation. In other words, it helps keep the calcium that is in the urine dissolved rather than settling out.
Both drugs are extralabel in these species, and our approach rests on clinical experience plus extrapolation from human medicine rather than controlled trials. I think it's important to say that plainly to families.
In guinea pigs, I think about the combination for any patient going to cystotomy, and especially for repeat stone formers. Starting it a few days before surgery, rather than after, is worth considering, since urinary citrate takes a few days to build in people. In rabbits, it's an option for sludge that keeps relapsing despite good diet, weight and analgesia work.
Monitoring matters more than the starting plan. We run a baseline minimum database before starting, recheck within the first several weeks with an eye on potassium, then recheck periodically while the drugs continue. In our hands neither drug has caused overt adverse effects, and we haven't seen clinical hypokalemia or hypercalcemia. Thiazide-associated hyperglycemia is described in people, so a glucose on the panel is sensible. Both drugs are usually continued long term.
For rabbits, the foundation is still cutting back alfalfa, encouraging water intake (many prefer a bowl), weight loss and exercise to keep sludge in suspension, and multimodal analgesia for the arthritis that keeps them from moving and posturing. Perineal dermatitis needs a sanitary clip and topical care. Ultrasound-guided cystocentesis avoids the large GI tract and the artifacts of a voided sample.
For guinea pigs, radiograph the whole perineum, add ultrasound when localization is unclear (including the seminal vesicles in boars), and culture the urine. For cystotomy closure I use Monocryl in a simple continuous pattern. In my experience, double-layer closures and PDS have been associated with more recurrence. And warn families early about recurrence and what repeat surgery means: adhesions, cost and repeated anesthetic risk.
If you're wondering whether HCTZ and potassium citrate make sense for a particular patient, or you have a guinea pig heading for a third cystotomy or a rabbit whose sludge keeps coming back, I'm glad to talk it through, including how we start and monitor these drugs. Reach out to Nashville's guinea pig and rabbit specialist: Dr. Logan Kopp, DABVP (Exotic Companion Mammal Practice), at Priest Lake Veterinary Hospital, (615) 361-4646. Referral details are on the referring veterinarians page.
This content is for educational purposes for veterinary professionals. It doesn't establish a VCPR or replace clinical judgment for an individual patient. Drug use and doses should be checked against current references and adjusted for each case.