Atomic Tabs has 20mg niacin (100% DV), and 5 hour energy has 30mg (150% DV). That is what gives the "flush" you are describing. I've never gotten a flush from Atomic Tabs, and I've not had anyone report they have either.
You may have a sensitivity to Niacin if 30mg gives you that reaction. For comparison, prescription Niacin is typically prescribed in doses ranging from 500mg to 2000mg.