I think it's for Corsodil (sp?) mouthwash. It says something along the lines of...
spitting blood
When you brush your teeth?
you might have gum disease
I was sat in traffic looking at it and had a sudden urge to sneak back at the dead of night to change the wording to...
Shitting blood
When you push?
You might have bum disease
I now laugh every time I drive past
Ps. This is a lighthearted thread I don't advocate vandalism or grafitti
Pps. I'm just laughing at the wording, not really at rectal bleeding