It can give a hypertensive headache which happens when your blood pressure rises too quickly. If you go to the ER they'd give you a pill to quickly lower your blood pressure. You won't miss the reaction when it happens, it hurts and you feel very very strange. The older the cheese is, the drier it is; I guess it also contains more tyrosine as it ages which is what causes the reaction. So some cheese might be old but not old enough to cause a problem but since there's no way to know how old cheese is, doctors tell patients to avoid it.