I've experienced skin flushing which can be so intense that uncomfortable isn't the word for it. I was taking Niacin to control cholesterol and you must be careful to ramp slowly up, then maintain a therapeutic dose without missing any doses.
If you do you have to drop the dosage back down or you invite skin flushing. Flushing is a crazy intense itching that occurs everywhere and cannot be satisfied. It only lasts for a minute or two, but it seems like an eternity. Also, the Niacin
can give you gout, which is when you exceed your body's ability to dissolve it and it accumulates in your joints as a crystalline substance and that's painful. I finally gave up on the Niacin and went with a Statin.