Patients with refractory overactive bladder (OAB) may be offered OnabotulinumtoxinA (Botox®) as one of the third-line options. Given the invasive nature of requiring cystoscopy, injections via a needle and local anaesthesia, it would be simpler, more convenient and more accessible if this could be delivered via instillation and not injection; indeed, this is a common question asked by patients. Unfortunately, a phase two, multicentre randomised controlled trial (RCT) has not reported the hoped-for outcomes. Reverse thermal hydrogel is a liquid at cold temperatures and a gel at body temperature; it was hypothesised that it would prolong the retention of active drug within the bladder to allow absorption via passive diffusion, a technology that has been used successfully with mitomycin for low-grade upper tract urothelial cancer. A previous small pilot randomised study had reported superiority over placebo but this phase two RCT on 607, mainly female, participants did not, and there were no reported adverse events either. It is possible that the large onabotA molecule is too large to diffuse across the urothelium without additional help, unlike mitomycin which is a small molecule.