This primary question needs to be answered before any discussion about vitamin Supplements.
ARE VITAMIN SUPPLEMENTS SAFE TO CONSUME?
This demands a complex answer because it depends on many variables. As a basic description, when properly designed supplement amounts added together with dietary food vitamin amounts are more or less within balance points for healthy body metabolism, then supplemented vitamins are safe. Unfortunately, since most supplement vitamins are concentrated, this is more of a challenge than it ought to be. 
Plus, the body has over time adapted many counter mechanisms to deal with unbalanced nutrient amounts. These often work quite well for a period of time, but then organs can be quickly overwhelmed and enter into a disease state.
These counter balancing mechanisms make it extremely difficult for Scientists to connect past deficiencies or excesses of vitamins to the present disease. Extra calcium without related protective nutrients can build up in artery walls and eventually lead to heart disease. In this case the protective nutrients are magnesium, vitamins D and K2, and 9-cis vitamin A. These act to bind calcium into bones and prevent calcification of arteries, plus they also help regulate blood sugar.
It can also be said that between these consumed groups; food, medicine, supplements, and illicit drugs, Vitamin Supplements are by far the safest group. There is still a lot of room for improved effectiveness and safety. Next is the reference showing results of a multi-year analysis of many studies. ref Mainly fat soluble vitamins plus vitamin C and folic acid are the ones they found suspect. Fat soluble vitamins can build up higher levels with continued consumption. Plus they get stored in fat cells.
Thus, multi-vitamin supplements can either be safe or not so safe depending on design formulas. Are the dosages within limits, are ideal formats for each vitamin used, and does it contain necessary synergistic nutrients to maximise vitamin performance? Of course this also means that this product has to factor in individual body characteristics and unique dietary considerations. The Scientists at NIH Office of Dietary Supplements have to generalize and compromise when setting daily vitamin recommendation amounts to serve the majority of people. Vitamin Manufacturers then use these recommendations to develop multi-vitamin formulas, but are always trying to find extras to gain a marketing advantage, like higher dosages. Thus by the time these compromised products reach the Public marketplace, is it realistic to assume they are a perfect match for your health needs, or just a compromised product in need of modifications to your individual health concerns.
Appropriate multi-vitamin formulas are rare and difficult to find. Plus, science has yet to even research and answer all the questions for proper vitamin design. Correcting factors to improve this situation represents the guiding principals behind the research information and suggestions in website articles.
Vitamin Safety has come under fire lately in many larger studies. Nil and even negative findings are resulting in Scientists and Media calls for the Public to stop taking vitamins. The studies are failing to find benefits to the vitamin forms and dosages used in the studies for reducing heart disease, cancer, and dementia. There are actually some truths in these studies, but it is the take away that Scientists are failing to properly see and report that needs to be communicated to the bewildered Public. What would happen if different forms and dosages were used? Or what if needed synergistic nutrients that compliment study vitamins were added?
There is a lot more coming on this safety topic, plus many of the other articles cover different vital aspects as well.
Here are some of the areas where excess vitamins and sometimes a deficiency too found in supplements may exhibit concerns:
- Vitamins can enhance or hinder absorption of other nutrients Excess calcium could block out other minerals like magnesium, zinc, and phosphorus. Vitamin E as alpha tocopherol can block out other vitamin E family members coming with food as well as vitamin K.
- Vitamins can block out other nutrients from carrier binding proteins or docking attachments Vitamin D has different forms. The non active and the active forms both combine with VDR, Vitamin D Receptors, and VDBP, vitamin D Binding Proteins. The two forms need to be in harmony.
- Vitamins can hinder natural cascade of metabolic processes Synthetic vitamin E has forms that do not exist in nature but can bind receptors without activating them. Just like the wrong key can go into a lock, but will not open it.
- Beta Carotene and Lutein, Carotenoids (which can become vitamin A) team up to protect eyes But, if an excess is ingested over time, the yellow orange color pigments they contain can lead to oxidative stress and damage in the macula as it becomes discolored, just like the skin and whites of the eyes can turn yellow orange from drinking too much carrot juice. The discoloration is a sign that the liver cannot process the colored carotinoids fast enough to eliminate and is storing them in the skin or whites of the eyes until processing can catch up. An observable sign to reduce intake. If discoloration is not the result of high carrot juice or carotenoid supplement intake, see a doctor as this could be sign of liver function.
- Some B complex vitamins exhibit toxic activities in both excess and deficiency. B6 is connected to nerve damage. Folic Acid to both cancer prevention OR stimulation of growth at different stages of disease.
- Fat soluble vitamins, vitamins A, D, E, K, Beta Carotene, etc, if taken every day at higher amounts can reach excess levels and overwhelm body processing. Lower dosages are preferred here or higher amounts need to be rotated with off and on days. This gives the body a chance to process and properly handle these amounts. This is not talking about toxic levels, but rather just slightly an excess. One area where this effect can be monitored is in the eyes. Certain carotenoids tend to accumulate in eye retinal tissues at the back of the eye and turn it slightly off color.