The best multivitamins have their vitamins and minerals in highly absorbable forms, like amino acid chelates and citrates rather than sulfates, carbonates or oxides.  Chelated mineral supplements can be absorbed 3 to 10 times more easily than the non-chelated forms.

Fat-soluble vitamins are available in dry or water-soluble forms for individuals who are sensitive to oils or who are on a lowfat diet. Fat is essential for the absorption of vitamins A, D, E and K, so a diet that is low in fat may increase the risk of deficiency of these vitamins. Supplementing with the dry forms of vitamins may prevent low levels.

In general, natural forms of vitamins are better than synthetic forms. In some cases, synthetic vitamins have been found to cause health problems rather than prevent them – as with synthetic vitamin E and beta-carotene. Natural forms may be slightly more expensive, but it is worth the investment as there are fewer toxic reactions or potential intestinal upsets.

When choosing a multivitamin product, consider the ODI for each vitamin; healthy levels should be obtainable with a reasonable number of capsules or tablets per day.

Also consider critical micronutrients for some disorders, like boron (bone disorders and arthritis) and vanadium (blood sugar and immune disorders). These are included in some vegetables and fruits, but if you do not eat enough you may need a multivitamin that includes them. Major vitamins such as A, B, C, E and minerals such as zinc are usually more important for most people than these micronutrients, but they can be a useful measure for gauging the quality or suitability of a multivitamin.

Some individuals may require vitamin K for bone health or proper blood clotting while others may need to avoid it. Patients on blood thinners or those with heart disease should not take vitamin K and should choose a multivitamin without it.

Most alternative health professionals recommend against taking iron supplements, especially for men and post-menopausal women. Most brands offer iron-free multivitamins or include just a small amount, within the 25% RDA range. Unless there are special needs, only children and premenopausal women should take supplements with a significant amount of iron.