The ideal in drug therapy is to achieve the right dose, of the right drug, for the right time, in the right patient. Choice of drug dosage regimen in order to achieve optimal drug concentrations in an individual patient is based on the likely pharmacokinetics in that individual. In the case of loading doses it is the volume of distribution (Vd) that is the major determinant, while for maintenance dosing the most important determinant is clearance (CL). The half-life is important for choice of dose interval, time to steady-state and time for drug removal from the body. While the broad approach is well defined, adoption of the principles in clinical practice is far from universal. This is exacerbated by the fact that the details of how Vd and CL are affected by different covariates are often not well defined. A logical approach would assist clinical practice and help direct further research. The initial doses chosen for an individual are based on an estimate of that individual's likely pharmacokinetic characteristics, usually by utilizing all known information about others in the population who resemble this individual. The population pharmacokinetic values are then adjusted for covariates such as age, weight, gender and any other characteristics that might make the individual different from the mean member of the population. Underlying this process are simple equations for loading dose (LD), which is based on Vd, and maintenance dose-rate, which is based on CL for drugs with first order elimination. The aim of the LD is to achieve an initial target concentration (Cinitial), whereas for the dose-rate it is to achieve a target steady state concentration (Css). (1) (2) Both Vd and CL are usually estimated after intravenous administration. If calculated after oral administration then the oral availability needs to be considered. For simplicity in this paper we will ignore oral availability. These equations can be adjusted to accommodate any factor known to explain some of the variance in Vd or CL. The main covariates of CL are shown in Table 1 and are similar for Vd. Some of these covariates may be interdependent and should not be accounted for twice. It would be useful if the effect of covariates could be incorporated into universal equations for dose-individualization. Table 1 Examples of covariates of clearance As loading dose is often a single dose only, Vd matters less clinically than CL. The discussion is therefore focussed on CL and maintenance dosing.