Are there any possible bad side effects of overdosing multivitamins?

Yes: First, stick w/reputable brand(s) b/c there's no fda requirement to demonstrate safety or efficacy, much less avoid contamination. Second, be careful about fat soluble vitamins (ade&k) which can accumulate in the body as opposed to water soluble ones which we just pee out. Also be aware that vitamins a & e have been associated w/cancer & other bad outcomes in specific populations.