Supplement manufacturers sell many vitamins not only as part of a multivitamin formula, but in addition, separate doses of varying amounts. For example, it’s common to find vitamin B12 in 3 individual doses: 500, 1000, or 5000 micrograms.
Food and supplement labels list the amounts of most nutrients as a percentage of a standard called the Daily Value. The percent is the amount of a nutrient in a food or supplement recommended as part of a 2000 calorie diet. As a general rule, a Daily Value of 5% or less indicates that the food is low in that nutrient and Daily Value of 20% or more indicates that it is high.
This brings to mind that in any nutrition course, you would be advised about the Tolerable Upper Intake Levels (ULs).
There are no values for ULs on supplement labels. These standards represent the maximum level of daily intake of a nutrient that is unlikely to pose of risk of adverse health effects to almost all individuals in a specified group. These are not recommended levels but rather levels of intake that can probably be tolerated. For some nutrients, data are insufficient to establish a UL value.
High intakes (megadoses) of vitamins and minerals are not recommended. A study found that long-term higher intakes of supplemental vitamins B6 and B12 increased the risk of lung cancer in men but not in women.
The recommended intake for B6 for adults is 1.3-1.7 milligrams/day.
The UL is 100 milligrams/day. The study reported that long-term use of over 20 milligrams a day increased lung cancer risk in make smokers.
The recommended intake for B12 for adults is 2.4 micrograms/day.
The UL has not been determined. The study found that long-term use over 55 micrograms a day increased lung cancer risk in male smokers.
The following article details the study and its possible association with lung cancer at high doses in men, especially smokers. It is important to tell your doctor what vitamin and mineral supplements you are taking to avoid any mega-doses unless there is a medical reason.