The premise of pharmacotherapy is the effective and safe use of drugs. In therapy, so-called standard doses are usually used. However, they are not optimal for all patients. There are numerous individual factors that make the use of a standard dose not always produce the intended effect. Therefore, the dosage in each patient should be considered individually, analysing the factors that may affect the fate of drugs in the patient’s body. Sometimes it is necessary to adjust the dose by reducing or increasing the dose, possibly changing the dosage range, modifying the time of taking the drug, or choosing its optimal form.