Nymph is right about antioxidants, but vitamins (iron containing, as most are) you DO NOT want to od on.
An iron overdose is easy to achieve with many medium to high content iron vitamins, not directly sure of doses though. Symptoms include fever, nausea, headache, shock and coma (which can occur as quickly as 1/2 hour after ingestion). These symptoms may even clear in a few hours and then return up to 24 hours later.
So again, if they're vitamins or anything iron containing, you don't really need it, so skip taking too much. More people die than you'd think, including a person I knew from school.