Pharmacogenetics is the science that allows different concepts to be applied to medical care. We define Pharmacogenetics as "the science that studies the actions and interactions between medicines in each person according to their genes". It studies the different responses that each person will have to the same medicine, while pharmacogenomics studies the effects of the medicine with respect to gene expression in general. When we read in the leaflets that accompany the medicines and check their characteristics all have a list of side effects, which are often reported in percentages, for example: "in about 5% of the population may cause dizziness and headache". With the knowledge provided by pharmacogenetics, in some cases the above sentence could be supplemented as follows: "Patients with the X gene polymorphism in the Y gene will produce dizziness and headache." Therefore, for patients with this polymorphism, it may be advisable to look for an alternative medicine, with similar therapeutic effects which does not produce these side effects. Pharmacogenetics has as one of its main objectives the prediction of the risk of toxicity and/or therapeutic failure when administering medicines to a specific patient. The ultimate goal is to prevent the toxicity and/or therapeutic ineffectiveness of a pharmacological therapy or, in other words, to optimize the benefit/risk ratio of a treatment in a personalized way. Pharmacogenetics allows us to:

1. Have a tool to practice a personalized medicine.

2. Get a better therapeutic response and less adverse or toxic effects.

3. Shorten treatment periods and, in hospitalized patients, reduce hospital stays by giving adequate medication to each patient.

4. Reduce the number of accidents and deaths from adverse reactions to medications and interactions in polymerization.

5. Reduce the economic and health costs of patients by ineffective medications.

6. Improve efficiency in Phase III and IV clinical trials.

7. Give scientific arguments to regulators before to accept or reject medicines.