Recurrent urethral obstruction in male cats: prazosin vs placebo — LIKELY (the treatment made no difference)

In male cats after urethral obstruction is relieved, does prazosin vs placebo/no treatment reduce recurrent obstruction?

Shelf #264 · gate-verified and displayed under the developer's blinded-trial rule

LIKELY (the treatment made no difference): In male cats after urethral obstruction is relieved, prazosin does not reduce recurrent obstruction compared with placebo. Two prospective blinded randomised placebo-controlled trials agree: Reineke 2017 (47 male cats, double-blinded; prazosin 0.25 mg/cat every 12 hours for one month) found recurrent obstruction in 4/26 (15%) on prazosin vs 3/18 (17%) on placebo over the medication period (P = 0.776), with no difference before discharge or at six months; Hanson 2021 (65 completing castrated male first-time obstructors, randomised and blinded; prazosin 0.5 mg every 12 hours for seven days) found recurrence in 11/37 (30%) on prazosin vs 5/28 (18%) on placebo within 30 days (P = 0.27). The retrospective Hetrick & Davidow 2013 signal that made prazosin routine was a comparison against phenoxybenzamine (30-day recurrence 18.18% vs 39.02%), not against placebo, and it is carded as context only. The stamp is LIKELY rather than PROBABLE because both blinded trials were small and Reineke's authors state theirs was underpowered — a very small benefit cannot be excluded, but no benefit has been shown and the only numerical trend favours placebo. Drafted Nov 10, 2026 (VetMed build, topic #264); all figures from abstracts fetched and verified at drafting.

Reineke 2017: prazosin made no difference to recurrent obstruction at any time point (the treatment made no difference)

Double-blinded, prospective, randomised placebo-controlled study (record's own abstract wording; MeSH includes Double-Blind Method)

Who: Cats: 47 consecutive male cats with urethral obstruction not associated with urinary tract calculi > 2 mm in diameter, at a university teaching hospital, randomised after obstruction relief to prazosin 0.25 mg/cat PO every 12 hours (n = 27) or placebo (n = 20) for one month, and monitored in hospital, weekly for a month, and again at six months.

Result: There was no difference in recurrent urethral obstruction (rUO) between prazosin and placebo before hospital discharge (2/26 [7%] vs 1/19 [5%], P = 1.00), during the one-month medication period (4/26 [15%] vs 3/18 [17%], P = 0.776), or at six months (7/19 [37%] vs 4/13 [31%], P = 0.811). There was also no difference in severity of lower urinary tract signs reported by owners at the 1-, 2-, 3- or 4-week follow-ups (P = 0.62, 0.68, 0.33, 1.00). Reported adverse effects of prazosin included lethargy, ptyalism, diarrhea, anorexia and malodorous stool.

Watch out: The authors themselves state the study was underpowered to identify differences in rUO, so this trial alone cannot exclude a modest effect — it is carded as one of two blinded placebo trials pointing the same way. Follow-up denominators shrink by six months (19 and 13 cats), and cats with calculi > 2 mm were excluded, so the population is obstructed male cats without larger stones.

Reineke EL, Thomas EK, Syring RS, Savini J, Drobatz KJ. The effect of prazosin on outcome in feline urethral obstruction. J Vet Emerg Crit Care. 2017;27(4):387-396. PMID: 28510274. Registry ID: none in the record. MeSH: Adrenergic alpha-1 Receptor Antagonists · Animals · Cat Diseases · Cats · Double-Blind Method · Incidence · Male · Prazosin · Prospective Studies · Urethral Obstruction

Hanson 2021: recurrent obstruction was 30% on prazosin vs 18% on placebo — no significant difference (the treatment made no difference)

Randomized Controlled Trial, Veterinary (record's publication type); randomised using a random numbers table, treatment given in a blinded fashion

Who: Cats: castrated male cats presenting for the first time with urethral obstruction at a referral setting (enrolled May 2014 to August 2017), treated with standardised anaesthetic, analgesic and indwelling urinary catheter protocols, then randomised to prazosin 0.5 mg PO every 12 hours for seven days or placebo, with 30-day telephone follow-up for recurrent obstruction; 80 cats were enrolled and 65 completed the study (37 prazosin, 28 placebo in the analysis).

Result: Sixteen of 65 cats (25%) experienced recurrent urethral obstruction (rUO) within 30 days: 11/37 (30%) on prazosin vs 5/28 (18%) on placebo, with no significant difference in frequency of rUO between prazosin and placebo (P = 0.27). Ten of the cats that reobstructed did so while still hospitalised. The authors conclude that prazosin 0.5 mg PO every 12 hours did not decrease the rate of rUO in this population versus placebo.

Watch out: The point estimate runs against prazosin (30% vs 18%) but the confidence around it is wide — 65 completing cats cannot exclude either a small benefit or real harm, and 12 enrolled cats were excluded for not receiving the full study medication. First-time, castrated, referral-population obstructors only; cats pre-treated with prazosin, phenoxybenzamine, steroids or NSAIDs, or with cystoliths on radiographs, were excluded.

Hanson KR, Rudloff E, Yuan L, Mochel JP, Linklater AK. Effect of prazosin on feline recurrent urethral obstruction. J Feline Med Surg. 2021;23(12):1176-1182. PMID: 33749375. Registry ID: none in the record. MeSH: Animals · Anti-Inflammatory Agents, Non-Steroidal · Cat Diseases · Cats · Male · Prazosin · Recurrence · Urethral Obstruction

Hetrick & Davidow 2013 (context): prazosin-treated cats reobstructed less than phenoxybenzamine-treated cats — the retrospective signal the blinded trials then failed to confirm

Retrospective case series — context card (not a placebo comparison)

Who: Cats: 192 male cats with urethral obstruction treated at an emergency and specialty center from 2004 through 2010, whose records were reviewed for treatment factors during initial urinary catheterisation and recurrence of obstruction at 24 hours and 30 days after catheter removal.

Result: Overall recurrent obstruction rates were 10.94% (21/192) at 24 hours and 23.57% (37/157) at 30 days. Recurrence developed in 10/140 (7.14%) prazosin-treated cats at 24 hours and 20/110 (18.18%) at 30 days, compared with 10/46 (21.74%) and 16/41 (39.02%) in phenoxybenzamine-treated cats — significantly lower with prazosin at both time points. Reobstruction at 24 hours was also lower with a 3.5F than a 5F urinary catheter (7/105 [6.67%] vs 11/58 [18.97%]). There was no association with duration of catheterisation, antimicrobials, meloxicam, or consistent pain medication.

Watch out: Context only, as labelled: this is a retrospective comparison against another alpha-blocker (phenoxybenzamine), not against placebo or no treatment, and drug choice was not randomised — sicker or easier cases may have been channelled to one drug. It explains why prazosin became routine; it does not outweigh the two prospective blinded placebo trials above, which found no benefit over placebo.

Hetrick PF, Davidow EB. Initial treatment factors associated with feline urethral obstruction recurrence rate: 192 cases (2004-2010). J Am Vet Med Assoc. 2013;243(4):512-9. PMID: 23902444. Registry ID: none in the record. MeSH: Adrenergic alpha-1 Receptor Antagonists · Analgesics, Opioid · Animals · Anti-Bacterial Agents · Anti-Inflammatory Agents, Non-Steroidal · Cat Diseases · Cats · Male · Meloxicam · Retrospective Studies · Secondary Prevention · Thiazines · Thiazoles · Urethral Obstruction

Synthesis: two blinded placebo trials, no benefit — prazosin does not reduce recurrent obstruction (the treatment made no difference)

Synthesis

Who: Synthesis of the cards above: Reineke 2017 (47 male obstructed cats, double-blinded randomised placebo trial), Hanson 2021 (65 completing castrated male first-time obstructors, randomised blinded placebo trial), and Hetrick & Davidow 2013 (retrospective context, 192 male cats).

Result: Across both prospective blinded trials, prazosin's recurrent-obstruction rates were statistically indistinguishable from placebo at every time point — 15% vs 17% over one month in Reineke, 30% vs 18% over 30 days in Hanson — and the one numerical trend (Hanson) favours placebo. The older retrospective preference for prazosin was against phenoxybenzamine, not against nothing, and non-randomised. The practical reading: after the obstruction is relieved, routine prazosin for preventing reobstruction is a habit the blinded evidence does not support, and the drug is not free of adverse effects (lethargy, ptyalism, diarrhea, anorexia in Reineke).

Watch out: Both blinded trials were small and Reineke's authors state theirs was underpowered; 'no difference shown in two consistent blinded trials' is the claim — a very small benefit cannot be formally excluded, which is why the stamp is LIKELY rather than PROBABLE.

Synthesis of Reineke EL et al. (PMID: 28510274), Hanson KR et al. (PMID: 33749375) and Hetrick PF & Davidow EB (PMID: 23902444). Registry ID as on the individual cards.

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