Drug dosage, or the amount taken over a particular period, is planned as per a regimen that also decides the frequency and route of drug administration. A dosage regimen aims to establish a target plasma concentration that has a maximal therapeutic effect and the least toxicity. To do so, a plot of a drug plasma concentration versus time is used to identify a therapeutic window during which the drug dosage is safe and effective. Such drug concentrations can be achieved through continuous IV infusions or fixed doses at regular intervals. The drug's plasma concentration also depends on the route of administration. Different routes–oral, intravenous, intranasal, or intramuscular, affect the drug's bioavailability and clearance profiles, altering its plasma levels. Additionally, a patient's body weight, age, gender, and medical conditions, such as kidney dysfunction, pregnancy, or heart impairment, influence a drug's therapeutic levels in the body. All such parameters are used to adjust the dosage regimen to achieve the desired therapeutic response.