Which vitamins should I be careful with if I try orthomolecular medicine?

Depends... The vitamins to be most cautious of are the fat soluble ones. The most important part of this approach is to work with a doctor who has knowledge and experience in this area. Ask what training they've had and how long they've been doing it. I prefer to do testing before prescribing anything. Having said that, there have been no deaths in the last 10 years from vitamins.