As far as simple meditation is concerned, it is like relaxation, it does not do any harm. However, you need more stamina to do the Advanced Meditation. It involves stimulating different energy centers or Chakras.
I have not come across anybody who has done our programs and become psychotic. However, there are possibilities, if they are already bi-polar or schizophrenic. Thus, we tell them to avoid doing the Advanced Course programs. We ask them to stick to the basics; simple meditation, yoga asanas and simple pranayama. That should be alright.
We ask people not to overdo the practices. Since people feel so good doing them, they find it so enjoyable and stimulating; sometimes, they tend to overdo it. They may start meditating for 6 hours to 8 hours. This is where the problem occurs; which is why you need proper training, and a good trainer to guide you to do it within limits. We need to do only as much as is prescribed. Overdoing anything is no good. It is like consuming a lot of vitamins at one time. It throws one off balance.
It is absolutely safe to meditate for 20 minutes, 30 minutes, or up to an hour. Then, people become a little spacey, or lose touch with reality. Such sort of things can happen to people if they overdo it.