[Superman]

I know a little something about employment law and your situation might not fall squarely into either category but I wouldn't doubt that you are essentially an independent contractor. In either case, it might be fun to sit down with the doc and say "Hey, my rate has not changed in six years and I know of another way to save you $25K. Similar to the last three times I brought significant savings to your office. I wonder if we could talk about both. The savings and the rate you pay for my services."

If he says yes, then just tell him about the drug savings. See what he says about your compensation rate.