NAC is one of the supplements used to help mitochondrial function. Mostly, doctors will not suggest the supplements until a pharmaceutical company patents a way of producing the supplement. Hence, it has taken years to get niacin, omega-3's and methylated B vitamins commonly suggested by doctors (Metanx, Deplin, Lovasta, Niaspan).

Interestingly, the ones often used for psychiatric purposes help mitochondrial function. The brain uses a lot of energy.

You may be interested in the following books:
Fix the Broken Brain - Mark Hyman
It's Not Mental - Jeanie Wolfson