Oxybutynin

Day 1: #oxybutynin is an anti-muscarinic drug approved in 1970s for incontinence & overactive bladder. #oxybutynin remains a 1st-line drug in UK. Side effects, especially dry mouth can lead to stoppage because immediate release (i.r) tablets can cause high levels of an active metabolite. Modified release (m.r) tablets/patches improve side-effects & concordance

Day 2: #oxybutynin is indicated for overactive bladder & neurogenic bladder from age 5 in UK. Dose age related. Adult i.r 5mg 2-3/day;can increase to x4/day. Modified release tablets available, oral solution, & transdermal patches (adults only).

Day 3: i.r fast oral absorption, low bioavailability, increased with food. High 1st pass metabolism & liver CYP3A4 breakdown produces an active metabolite. This peak > anti-muscarinic SEs. M.r > stable levels & patches bypass liver metabolism;both improve tolerance. Clearance primarily hepatic. T½ ~2hrs; prolonged in elderly

Day 4: #oxybutynin competitively inhibits acetylcholine (ACh) action on M1,2,3 receptors. Blocking ACh causes relaxation of bladder smooth muscle, increased capacity & reduced urgency/frequency. #oxybutynin has selectivity for M1 & 3, so high impact on salivary secretion (M1 receptor). Can take weeks to work while the bladder adapts