A multi vitamin won't be nearly enough. In the book he advocates mega doses that are 100x-10,000x what you'll get in a multivitamin. For the 3% or so of the population that couldn't process B3 properly, they needed insane megadoses for the rest of their life. Many of them were hardcore alcoholics though so they must have had all kinds of damage to their liver and lord only knows what else.
http://orthomolecular.org/library/jom/1972/pdf/1972-v01n01-p046.pdf https://paulingblog.wordpress.com/2014/05/14/the-decline-of-orthomolecular-psychiatry/
are good starting points, and the book I read was https://www.amazon.ca/Orthomolecular-Psychiatry-Schizophrenia-David-Hawkins/dp/0716708981