Blood concentration levels can be used to determine therapeutic and safe dosing levels for a drug. Therefore, it is important to know the half-life of the drug in order to calculate the frequency of dosing required to reach and maintain therapeutic levels. The time taken to reach a steady-state concentration is six times the drug’s half-life. During clinical trials, blood samples will be taken from patients at regular intervals to determine the concentration of the drug in the blood. This will help determine the proper dosing regime in order to get the ideal concentration of drug in the blood. The drug can induce synthesis of metabolic enzymes, which will result in increased metabolism of the drug. Physical dependence occurs when a patient needs the drug in order to feel normal. If the drug is withdrawn, uncomfortable withdrawal symptoms may arise, which can only be stopped by retaking the drug.