What happens to the leftover Botox?

I've been seeing a neurologist for Botox for migraines. He orders 300 units from my pharmacy and injects 150. What happens to the rest? He never lets me use it the next time and whenever I ask questions, no one ever gets back to me. Is this a standard practice? I pay about $240 per every 100 units, and that's after insurance... that's over $700 to the specialty pharmacy. I'd sure not like to be spending more than I have to when it's so costly.