Introduction to Drug Distribution
Once a drug is absorbed into the systemic circulation, it will distribute over different tissues. This will lead to useful pharmacological effects in case this tissue is the target tissue, but can also lead to unwanted side effects is drug accumulates into other organs.
Knowledge on how a drug distributes among other tissues is necessary for proper dose determination, since there will be a balance/ratio in which the drug is present in the plasma and in the target tissue that will initiate a response. Thus, the concentration in the plasma is somehow related to the concentration at the target side, and therefore related to the pharmacodynamic effect.
Since it is inconvenient or sometimes simply impossible to measure concentrations at the site of action (e.g. at the dopamine receptors in the brain), blood plasma concentration levels are used instead, which should provide an indirect estimate of the concentrations at the target organ, tissue, or even receptor. Although the concentrations in the blood plasma can be very much different from the concentrations at the brain, the ratio in which distribution between plasma and other tissue takes place is important. This relationship is explained in the topic volume of distribution.