Unless, of course, it's a good one. How do you know your vitamins or supplements are worth their money? Take a look at some of the things I've shown on the label below (this label's taken from a popular Multivitamin, with 2 minor changes and no name to protect its identity)!
Many multivitamins don't give you value for your money because the types of ingredients may be cheap and poorly absorbed or possibly toxic, while the doses may be too low to be effective. Cheap supplements are often cheap for a reason; manufacturers use cheap binders and fillers (the 'non-active' ingredients that shape the tablet and hold the active ingredients together). These may reduce breakdown of the tablet and absorption of the ingredients, reducing your benefit and bang for your buck.
An easy way to check the quality of your multivitamin is to look at the tablet coating; if it's a single colour (Eg; green, yellow, pink, red, brown or white), then the coating is probably an oxide compound (Eg; red oxide). These compounds are great for painting driveways, but when used in tablets, often irritate the stomach and reduce the absorption of the other tablet ingredients. Many tablets also contain lactose as a binding agent, or may include gluten. If you're lactose intolerant or have wheat intolerance or coeliac disease, you my find yourself with an upset stomach or bowel after taking your supplement, and may not be absorbing the important nutrients.
Ideally, your multivitamin should be either a capsule, allowing excellent absorption, or a speckled tablet with no oxide coating.
At the end of the day, if you're wondering what supplements are best for you, it's worth your money and time talking to a Naturopath who can advise you of what's relevant, safe and effective for your ongoing health.
And those recent 'clinical trials' that prove multivitamins don't work? They used a junk quality tablet similiar to the one I've illustrated above, and failed to mention that the supplement also contained aluminium, a known neurotoxin! Go figure.