Actually, if you have less than, I think it's 36 grams (an ounce is a little over 28 grams) it's considered for personal use in the decriminalization plan (which may or may not be in force right now, they've flip flopped so often I'm not sure anymore), not for traficking.
Now, anyone who isn't Donald Trump tends to buy in 1/8 (3.5 g), 1/4 (7 g) or 1/2 (14 g) ounces (and that last is pushing it for most people). It seems to me that the only people carrying a full ounce around with them are the trafickers who keep their stash with them in case there's a raid on their place while they're out. So, why they set the 'personal use' amount as high (if you'll excuse the pun) as they did is beyond me.
Euw. I think I know a tad too much about this.
As for medicinal pot, severe arthritis: severe pain and chronic pain are on the list here in Canada. Check out this page Medicinal Marijuana
for qualifications of approval.