Choosing the appropriate multi-vitamin for a person is important so that its maximum benefits could be obtained. TDM (Therapeutic Drug Monitoring) usually does this when time permits and the person is willing to undergo the procedure.
In therapeutic drug monitoring, drugs are administered to a certain person and observed for its therapeutic effect. The drug dosage is adjusted until such time that the dosage is therapeutic and neither sub-therapeutic nor toxic to the person. 
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A more practical way is to take into account all of the factors that may affect the ingestion of the drug. In this process there are several factors that should be taken into consideration.
First and foremost is the physiologic make-up of the person. The “organ function” should be considered first. Does the person have a dysfunctional liver or heart? Is his excretory organ functionally well?
The vitamins for a person with a heart ailment should be one that has no side effects on his heart; likewise the vitamins of persons who have dysfunctional livers would have to have no toxic side-effects on his liver. Organs that are functioning normally would ensure that the purported boosting effects of the multi-vitamins would come about.
Second, the age of the person. Very young children and very old people usually have a slower body metabolism. Young people in their teenage years have the strongest. That is why vitamins are not commonly given at birth but only after 2 months. For geriatric (old) patients, the same principle applies too. The easily metabolized vitamin is a good choice.
Third, is the weight or body mass of the person. The higher the weight and body mass, the stronger would be the dosage. There are vitamins which have a higher concentration than others.
These maybe selected for optimum results for those with higher body mass.
Fourth, the economic status of the person. 
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The question: “Could he afford the prolonged maintenance of the multi-vitamins? If not, then cheaper alternatives should be selected. It is of no use if the person just takes the vitamins for a week then stops because he cannot afford to buy.
Fifth, the effectiveness of the multi-vitamins as documented by people who have used them. The best advertisement of a product is still through word of mouth from actual users.
Sometimes, it is only through trial and error that one would be able to choose the correct multi- vitamins for himself/herself. It is still an individual
