Pharmacodynamics is the word employed to describe the effect that a drug has on the body. It finds out whether the drug is going to be efficient in the treatment of disease with minimum side-effects. The initial step leading to a drug response is binding of a drug to its receptor. This drug receptor interaction is followed by the sequence of intracellular events which outcomes in the response of the cell to drug.

Pharmacokinetics is the word employed to describe the effect the body, its organs and tissues has on the action of a drug once it is administered. It is not usual for the similar amount of drug to be finally available at its site of action. As soon as the drug enters the body, the body will act on the drug to modify its concentration and in most of the cases its chemical structure. In order to be able to use drugs wisely and at proper doses we require understanding the pharmacokinetics of the drug. To exert an effect on the body, a drug should reach its target tissue in a proper form, and in adequate concentration, to initiate a particular therapeutic effect. Once the pharmacokinetics of a drug is known it is then possible to find out:

• The doses (or concentrations) to be given to the patient.
• The most efficient route of administration.
• How often the dose has to be given in order to get maximum therapeutic advantage for the patient.

The exclusive characteristics of each patient can affect both the pharmacokinetics and pharmacodynamics processes comprised in the drug action. They can as well find out the patient’s response. Sources of individual variation comprise:

• Interactions between the drugs if the patient is getting more than one drug at similar time.
• Physiological variables like age, sex and weight.
• Patho-physiological variables related to the disease (especially reduced function of kidney and liver, which are the main organs of drug elimination).
• Genetic variables that can modify the metabolism of drugs and predispose the patient to exclusive side-effects which can’t be predicted.

Whenever we consider all of the possible uncontrolled variables related with the treatment of an individual patient – like the patient’s disease and the multiple pharmacological effects of most of the drugs – it is simple to believe that any attempt at rational drug therapy is not possible. To the inexperienced and untrained student initial encounters with the drug treatment might appear as the best guess on the part of clinical team backed up by years of clinical experience of what drugs have worked in the past on patients with an alike illness. Luckily, this is not true. Of all branches of practical medicine it is perhaps the therapeutic feature of patient care with drugs which is most amenable to objective research. For illustration we can:

• Intervene the patient’s disease with a drug.
• Objectively measure the response to drug.

Use the data obtained from one group of patients to build up and inform good practice for the use of the drug in other patients.

Whenever we administer a medicine to a patient the active drug itself is only one variable in the complex pattern of events which will determine how that individual patient responds. This is important, therefore, that we spend time considering all of the factors that might happen to our therapeutic objective among the writing of the prescription and the patient’s final response to the drug.

By using the above information and also information from your own sources, describe the difference between the Pharmacodynamics and Pharmacokinetics in detail.