if you have any medical condition you can basically say the weed is an effective treatment for it and that you are already using it to treat this. depending on the doc you'll either have to get another doc's note or you'll get it right there on the spot.
for me, I have years of medical records and a permenantly injured arm with a fat scar on it, it cracks and shit when I move it and I only have 35% movement available in it.
so when I go to sign up for my script, all my evidence is right on my body. though I also have the paperwork to back it up.