Substitution Permissible

In my state, when a prescriber endorses a prescription on the "substitution permissible" line that means we can freely give generic instead of brand.

Recently, however, a patient argued with me that it meant I could also change the strength. 

Cialis is promoting their 5mg daily tablets by printing all sorts of newspaper ads telling people they can get a free 30 day supply of the 5mg if they can get a prescription from the doctor. So a dude shows up with a prescription for Cialis 20mg and tells me I can give him the 5mg because the doctor signed on the sub line. 

I told him no, that's not what it means. However, I can certainly call the doctor and get a verbal authorization to change it.

"No, don't do that!" he said, grabbing the Rx out of my hand.

Hmmmm.... I'm thinking maybe he really did know what "substitution permissible" means...