Ritalin (methylphenidate) makes my eye swell up and then i have to mix ritalin (methylphenidate) with a sleeping tablet to stop this. why does this happen?

Allergic?: Not sure, but sounds like you are allergic to ritalin. Allergies classically cause swelling of a part of the body. The reason why your "sleeping pill" helps is because most over-the-counter sleeping pills are a generic form of benadryl, (diphenhydramine) just advertised as a sleeping pill. Benadryl (diphenhydramine) treats allergies, that's probably why it helps. I would stop Ritalin completely.