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Nowadays, many drugs are widely used for toddlers and children without approval. But those drug formulations prepared for adults may not be appropriate for child patients. The drug levels in blood (TDM) are measured periodically in our clinics. Advanced investigations (phenotyping/genetics etc.) are carried out manly if incorrelations exist between the drug levels in blood and the clinical responses, and if required for other reasons. The results of our investigations show that bulk of our clinical population have results differing from the average. In several countries, TDM measurements and phenotyping are widely used for pediatric population. Unfortunately this is not valid for our country. This presentation aims to share our relevant clinical data, and to have discussed the matter in a scientific platform. Drug use for children; there are pharmacogenetic and pharmacodynamic differences among children due to the physiological differences at their development stages. Therefore it is essential to consider the development stage of a child when any drug is used. Newborn stage, toddler stage, childhood stage and adultery stage are different for their biological benefit from the drug which changes pharmacogenetic and pharmacokinetics of a drug so that the main and side effects will change. Determining the pharmacogenetic differences amongst various communities and ethnical groups is another area of implementation of this approach. Due to the wide existence of some polymorphic variants in some communities that may be more sensitive to the pogress of illness and drug treatment. Dividing the patients into different genotypical groups and applying different sausages or even drugs for the same illness is a treatment in clinical practice today. In near future, several new indications are expected to be on the agenda. Emerging of the ultra quick technologies, such as Microerey and chip technologies, may soon take place in routine clinical applications in determining the polymorphic genetical characters of the patients prior to their medical treatment.