In the old days (just a few years ago), I suggested most people take a multivitamin. I myself took a daily multivitamin for most of my life - my mother thought it was a good idea, and when I became a doctor, it still seemed reasonable.
Unfortunately, most studies find no benefits from taking multivitamins, and those that do suggest a benefit are generally poorly done studies with insufficient statistical power to prove anything.
Imagine my surprise and chagrin when I began reading studies suggesting HARM from certain components of multivitamins.
B-vitamin therapy seems to worsen kidney function in diabetics (JAMA, 303:1603, 4/28/10)
Folic acid supplementation seems to increase the risk of lung cancer, and perhaps for other cancers as well (JAMA 302:2119, 11/18/09)
Beta-carotene (a precursor to vitamin A) seems to increase the risk of lung cancer in smokers.
Beta-carotene, vitamin A and vitamin E seem to increase the risk of death - very small "relative risks" of about 1.04 to 1.16 (JAMA 297:842, 2/28/07)
Vitamin E supplementation in those treated for head and neck cancers led to INCREASED progression of disease (J Natl Cancer Inst 97:481, 4/6/05)
Vitamin E was associated with an increased risk of heart failure in patients with heart disease (JAMA 293:1338, 3/16/05)
There are many other studies I could highlight, but the bottom line is that there is at least SOME concern that certain elements in multivitamins (notably folic acid, vitamin A/beta carotene, and vitamin E) might increase your risk of certain problems (cancer, heart problems) and might increase your risk of death. These risks are very small relative to the risks of smoking, not exercising, and being overweight - but it is much easier to simply NOT take a multivitamin compared to trying to lose weight!
A study published in 11/12 DID find a SLIGHT reduction in the incidence of (but not death from) cancer among healthy men. However, that is only one study among many, and the benefit was very small. If you would like to read this study or the editorial, click on these links: Study and Editorial.
There is absolutely no question in my mind that a healthy lifestyle, including a healthy diet, is your most important tool in maintaining good health. At this point, I see enough questions about some components of multivitamins that I no longer recommend them. I do feel that vitamin D remains a safe and important vitamin that you cannot get from the average healthy diet (it is present in relatively small amounts in some dairy products).
So - bottom line - I suggest you consider stopping your multivitamin in favor of taking vitamin D 600 to 800 IU per day. Eat at least 5 servings per day of fruits and vegetables, exercise every day for 30 minutes if possible, and limit animal fats in your diet (plant fats, like those found in nuts and olive oil, are OK in moderation). The one exception to multivitamins is in women who might become pregnant - for the health of the baby, these women SHOULD take a daily multivitamin containing at least 400 mcg of folic acid.
What about calcium?
Many guidelines recommend adults get around 1200 mg of calcium in their diet, and if they do not, guidelines suggest taking a calcium supplement. I have recommended calcium supplements for years. However, a recent "meta-analysis" (a study that combines many prior studies into one big statistical study for better analysis of small risks) found that calcium supplements in excess of 600 mg per day appear to increase the risk of heart attacks. At this time, I suggest people TRY to get ALL of their calcium from diet (3+ dairy servings per day), rather than supplements. IF this is not possible, I suggest taking the equivalent of ONE Tums EX (contains 750 mg of calcium carbonate - which means 300 mg of elemental calcium) once or twice per day.