Chemicals can have pharmacologically relevant properties and effects. 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). Systems, receptors and ligands Pharmacology is typically studied with respect to particular systems, for example endogenous neurotransmitter systems. The major systems studied in pharmacology can be categorised by their ligands and include acetylcholine, adrenaline, glutamate, GABA, dopamine, histamine, serotonin, cannabinoid and opioid. Molecular targets in pharmacology include receptors, enzymes and membrane transport proteins. Enzymes can be targeted with enzyme inhibitors. Receptors are typically categorised based on structure and function. Major receptor types studied in pharmacology include G protein coupled receptors, ligand gated ion channels and receptor tyrosine kinases. Pharmacodynamics Pharmacodynamics is defined as how the body reacts to the drugs. Pharmacology models include the Hill equation, Cheng-Prusoff equation and Schild regression. Pharmacodynamics theory often investigates the binding affinity of ligands to their receptors. Medication is said to have a narrow or wide therapeutic index, certain safety factor 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. The effect of drugs can be described with Loewe additivity which is one of several common reference models. Pharmacokinetics Pharmacokinetics is the study of the bodily absorption, distribution, metabolism, and excretion of drugs. When describing the pharmacokinetic properties of the chemical that is the active ingredient or active pharmaceutical ingredient (API), pharmacologists are often interested in L-ADME: Liberation – How is the API disintegrated (for solid oral forms (breaking down into smaller particles), dispersed, or dissolved from the medication? Absorption – How is the API absorbed (through the skin, the intestine, the oral mucosa)? Distribution – How does the API spread through the organism? Metabolism – Is the API converted chemically inside the body, and into which substances. Are these active (as well)? Could they