For bladder spasm, if I read it right, the AUA guideline for intravesical BCG/Chemotherapy treatment states
"Anticholinergics may be prescribed if the patient complains of bladder spasms." if the patient has the problem of retaining the agent. Refer to Chapter VII Patient Assessment, Section A Physical Exam, Paragraph 5.
www.auanet.org/guidelines/intravesical-administration-of-therapeutic-medication
In terms of dose deduction, there are mixed messages among urologists. But it indicates continuining with lower dose seems to be longer recurrence free period than stopping when the full dose becomes not tolerable.
There was a virtual meeting by well-known urologists at AUA 2020 meeting. Though the title was "what do during BCG shortage for high-risk NMBIC patients?", it talks about the efficacy of reduced dose frequency. Dr. Seth Pau Lerner of Baylor College of Medicine, TX said appropriate cytokine response may be observed with very low disease as little as 1/100th of a standard treatment dose. Dr. Ashish Kamat of MD Anderson highlighted that the duration of therapy (1 versus 3 years) appears to be more influential than dosage (1/3 versus full dose) as highlighted in data from the EORTC 30962 trial. Below the result of the EORTC 30962 trial.
5 year disease-free rate ( same as recurrence-free)
1 year @ 1/3 dose 54.5%
1 year @ full dose 58.8%
3 year @ 1/3 dose 62.69%
3 year @ full dose 64.2%
www.urotoday.com/conference-highlights/aua-2020/aua-2020-bladder-cancer/122538-aua-2020-what-to-do-during-bcg-shortage-for-high-risk-nmibc-patients.html 
