Researching over 30 000 women aged 49-83, a study found that in women with no history of cardiovascular disease, multivitamins reduced heart attack risk by 27%. 

The results were even better for long-term users of multivitamins.  The women who used multivitamins for at least five years had their heart attack risk reduced by 41%. 

Possible explanations for this include:
* the antioxidant properties of multivitamins reduce the formation of plaques that clog up the blood vessels;
* Homocysteine, which promotes formation of plaques in the blood vessels, can be lowered by certain vitamins;
* Multivitamins often contain cardioprotective minerals such as magnesium which relaxes blood vessel walls and prevents inflammation.

(Rautianen S, et al. Am J Clin Nutr 2010)