DRUG

Drug (French: Drogue-a dry herb): It is the single active chemical entity present in a medicine that is used for diagnosis, prevention, treatment/ cure of a disease. This disease oriented definition of drug does not include contraceptives or use of drugs for improvement of health. WHO(1966) has given a more comprehensive definition-"Drug is any substance or product that is used or is intended to be used to modify or explore physiological systems or pathological states for the benefit of the recipient."
The term 'drugs' is being also used to mean addictive/abused substances.

Pharmacokinetics

Pharmacokinetics (Greek: Kinesis-movement) is What the body does to the drug.
It refers to the movements of the drug in and alteration of the drug by the body; includes absorption, distribution, binding/ localization/ storage, biotransformation and excretion of the drug, eg. Digoxin is ~70% absorbed orally; 25% bound to plasma proteins; localized in heart, skeletal muscle, liver and kidney; widely distributed (volume of distribution ~6L/kg); a small fraction is metabolized in liver to inactive products and is primarily excreted unchanged by glomerular filtration in kidney, has a total body clearance of ~150mL/min and a plasma half life (t1/2) of ~40 hours.

Pharmacodynamics

Pharmacodynamics (Greek: dynamis-power) : It refers to What the drug does to the body.
This includes physiological and biochemical effects of drugs and their mechanism of action at macromolecular/ subcellular/ organ system levels, eg. Adrenaline interacts with adrenoceptors resulting in G-protein mediated stimulation of cell membrane bound adenylyl cyclase which further results in increased intracellular cyclic 3',5' AMP that results in cardiac stimulation, hepatic glycogenolysis and hyperglycemia, etc.

PHARMACOLOGY

PHARMACOLOGY:

(Greek: Pharmacon-drug; logos-discourse in)

Pharmacology is the science of drugs. In a broad sense, it deals with interaction of exogenously administered chemical molecules (drugs) with living systems. It encompasses all aspects of knowledge about drugs, but most importantly those that are relevant to effective and safe use for medicinal purposes.  For many years, most drugs were crude natural products of unknown composition and limited efficacy. Only the overt effects of these substances on the body were rather imprecisely known, but how the same were produced was entirely unknown. //over the past 100 years or so, drugs have been purified, chemically characterized and a vast variety of highly potent and selective new drugs have been developed. The mechanism of action including molecular target of many drugs has been elucidiated. This has been possible due to prolific growth of pharmacology which forms the backbone of rational therapeutics.

The two main divisions of pharmacology are: Pharmacodynamics and Pharmacokinetics.

Ref.: "Essentials of Medical Pharmacology, fifth edition, by KD Tripathi"