Sorry you had the scare. Yes, mesalamine turns urine darker colors, from orange to brownish. Adding bleach to mesalamine instantly turns it dark brown to purple. Mesalamine can darken your pee, I randomly pee a darker brown to orange color and that's also a known affect. Mesalamine when exposed to air can change to a dark brown to purpler.
You're best to flush the toilet after each usage, and hitting it with the toilet brush cannot hurt!
> The use of sodium hypochlorite bleach as toilet detergent is related to a red-brown urine discolouration in
> patients taking mesalamine.It seems that the red-brown discolouration relates to a harmless
> reaction.Polymerisation of mesalamine and/or metabolites could be a theoretical mechanism of
> this discolouration.
https://www.ncbi.nlm.nih.gov/pmc/articles/pmc5005790/