Is it helpful to take multivitamins if I have ADHD & OCD, & I'm taking ritalin & Zoloft (sertraline) & omega-3 supplement (efalex active 50+)? And is it safe?

Probably; yes. I believe it is beneficial for most people to take multivitamins. It's also probably useful for anyone taking most pharmaceuticals. Studies show mutivits. Can have a positive benefit on mood, esp. Those high in b complex: http://bit.Ly/1lu4aou multivitamins are very safe- see http://bit.Ly/18xyule there is no guarantee multivitamins will help but an excellent chance a quality one can be of value.