[edit] Divisions
Pharmacology as a chemical science is practiced by pharmacologists. Subdisciplines include
clinical pharmacology - the medical field of medication effects on humans
neuro- and psychopharmacology (effects of medication on behavior and nervous system functioning),
pharmacogenetics (clinical testing of genetic variation that gives rise to differing response to drugs)
pharmacogenomics (application of genomic technologies to new drug discovery and further characterization of older drugs)
pharmacoepidemiology (study of effects of drugs in large numbers of people)
toxicology study of harmful effects of drugs
theoretical pharmacology
posology - how medicines are dosed
pharmacognosy - deriving medicines from plants

[edit] Scientific background
The study of chemicals requires intimate knowledge of the biological system affected. With the knowledge of cell biology and biochemistry increasing, the field of pharmacology has also changed substantially. It has become possible, through molecular analysis of receptors, to design chemicals that act on specific cellular signaling or metabolic pathways by affecting sites directly on cell-surface receptors (which modulate and mediate cellular signaling pathways controlling cellular function).
A chemical has, from the pharmacological point-of-view, various properties. Pharmacokinetics describes the effect of the body on the chemical (e.g. half-life and volume of distribution), and pharmacodynamics describes the chemical's effect on the body (desired or toxic).
When describing the pharmacokinetic properties of a chemical, pharmacologists are often interested in LADME:
Liberation - disintegration (for solid oral forms {breaking down into smaller particles}), dispersal and dissolution
Absorption - How is the medication absorbed (through the skin, the intestine, the oral mucosa)?
Distribution - How does it spread through the organism?
Metabolism - Is the medication converted chemically inside the body, and into which substances. Are these active? Could they be toxic?
Excretion - How is the medication eliminated (through the bile, urine, breath, skin)?
Medication is said to have a narrow or wide therapeutic index or therapeutic window. This describes the ratio of desired effect to toxic effect. A compound with a narrow therapeutic index (close to one) exerts its desired effect at a dose close to its toxic dose. A compound with a wide therapeutic index (greater than five) exerts its desired effect at a dose substantially below its toxic dose. Those with a narrow margin are more difficult to dose and administer, and may require therapeutic drug monitoring (examples are warfarin, some antiepileptics, aminoglycoside antibiotics). Most anti-cancer drugs have a narrow therapeutic margin: toxic side-effects are almost always encountered at doses used to kill tumors.