Great product. Years ago my GP prescribed Nicotinic Acid for my very poor circulation and freezing feet. At the time I hadn't realised that this is just plain Niacin.
About half an hour after you take it, you'll feel a warm feeling, almost like a burning sensation, in your face/head/ears and it gradually spreads to the rest of the body. Some people don't like the feeling (feels like little "explosions" inside), but for those of us who are freezing most of the time, it's a relief to finally feel your toes again.
Note that Niacin comes in two forms, Nicotinic Acid and Niacinamide. Nicotinic Acid is the one that gives you the warm flush feeling, not the other one.