An IC (interstitial cystitis) patient knows they need to drink water that has gone through a filter to remove all impurities. Otherwise those impurities could cause them to have a bladder flare. The pain of a bladder flare has been compared to stage 4 cancer. I’ve never had cancer, so I compare my flares to being stung by bees over and over.
I drink reverse osmosis water only. Water that goes through a filter is still not pure enough for my picky bladder. I never drink mineral water or tap water, it is just not worth the pain it might cause. But some IC patients find that filtered water is okay for them.