Is it normal miss a period for 3 months?

I was talking to one of my friends about late periods and she said not to worry, shes missed periods for 3 months several times, and up to 6 months. I immediately said there must be some kind of underlying medical issue, I've never heard of it etc...she said shes met loads of other girls who had the same thing, been to 2 doctors and theyve said everythings fine etc... I'm not talking about a young girl, I'm talking about a 23 year old. My instinct is that shes lying, in all my sex education etc at school I've heard that missing even one period if not pregnant or a young teenager whos recently started menstruating is a serious cause for concern...so has anyone else missed a period for 3-6 months? Even when they were young and their periods were irregular? I don't think it's normal...