Was on ritalin (methylphenidate) for 15 years. now have tachycardia daily, dizziness, balance and thinkibg issues. long term effects of ritalin (methylphenidate)?

No: not much is known about any long term side effects of Ritalin (methylphenidate)

Wouldn't think so: I wouldn't tag the Ritalin as the cause of your symptoms. Fifteen years of use in your childhood does not translate into vague health issues decades later, You need a thorough evaluation by your doc to find a treatable source of your problems. I would schedule it immediately.