Niacin, also known as nicotinic acid, is one of the products produced when nicotine is burned. In other words, it is one of the 'dangerous chemicals' in tobacco smoke, so there is 'no safe dose' of...
OTC antihistamines, especially ones that say "non-drowsy," are pretty safe. They can have side effects, but these usually only appear with older adults. As for "too much" I would guess an unusually...