For almost everyone, multivitamins and supplements are not necessary, do not work, and are probably harmful.
Taking vitamin A dramatically increases the risk of developing lung cancer in smokers.
Excess intake of vitamin E increases mortality (in other words, you are likely to die sooner), increases rates of heart failure, and significantly increases the risk of prostate cancer.
Taking calcium supplements actually increases the risk of suffering a hip fracture, as well as increasing the risk of death from cardio-vascular disease.
Taking anti-oxidants increases your risk of getting colon cancer.
If you are pregnant, a dark-skinned person who lives in a cold or dark environment, a diabetic who takes metformin, or if you have been on high dose of a proton pump inhibitor for more than a year, you should talk to you doctor (a real medical doctor, not a scammer like a chiropractor or naturopath), who will check, and if necessary advise about possible short term supplementation.
For everyone else, unless you have been diagnosed with a specific deficiency, vitamins and supplements are more likely to harm you than do you good. And yet people in the USA spend more than $32 billion per year on them.