Niacin in prescription strength? Just take more of the OTC. Easy, cheap, and the flush is good for you. Start with 500 mg and work your way up. Less if you're afraid of the flush.
I have to agree with the iodoral statement. I find I have to take the other supplements that go with it though. Especially the b vitamins. My feet are generally warm in the winter now. They’ve never been warm before, even in the summer.