Thanks for your query.
Unfortunately there are no medications available to" strengthen" the bladder wall.
It is unfortunate that you are not able to take dariten.
I would suggest the following non pharmacological therapy which has been found useful:
a) Weight loss
if your BMI is high
b) Avoid tea, coffee, colas, alcohol as these increase bladder irritability
c) Strengthen your pelvic muscles by doing kegel exercises
d) See a gynaecologist to rule out prolapse etc.
Hope this helps