Overview: Drugs are synthetic or natural substances used in the treatment, cure, prevention or diagnosis of disease or used to enhance physical or mental well-being. [1] Drugs can be everyday substances (caffeine, nicotine, ethyl alcohol); illicit substances (cannabis, heroin, cocaine) and food additives. [2] The study of drugs – what they are, how they work, and what they do – is called pharmacology. Pharmacology includes: • Pharmacodynamics: what a drug does to the body (includes biological effects and mechanism of action) • Pharmacokinetics – what the body does to a drug (includes absorption, distribution , metabolism and excretion of drugs and their metabolites) The most common ways in which a drug can produce its effects are shown in Figure 2. [3] Drugs that enter the body stimulate certain receptors, the Ion channels act on enzymes or transporter proteins. • Agonists – chemicals or drugs that activate receptors and produce a response. Agonists activate receptors for endogenous mediators – for example, salbutamol is a β2 adrenergic receptor agonist. The side effect may be excitatory (e.g. increased heart rate) or inhibitory (e.g. relaxation of airway smooth muscles). Nicotinic acetylcholine receptor agonists exert an inhibitory effect causing a lasting depolarization of the neuromuscular junction and therefore an inactivation of the Na+ channels which trigger the action potential. Agonists have affinity and efficacy. • Antagonists – chemicals that combine with the receptor and block the action of the emitting substance. Neutral antagonists block the effect of the agonist. There are two types of antagonism: competitive (reversible and surmountable) and non-competitive (irreversible and insurmountable). Competitive one...... middle of paper ......11): 1984-9.[14] Hernández F, Avila J. Tauopathies. Cell. Mol. Life sciences. 2007;64(17):2219–33. doi:10.1007/s00018-007-7220-x[15] Dorlands Medical Dictionary. No date. Available at -http://web.archive.org/web/20080130031824/http://www.mercksource.com/pp/us/cns/cns_hl_dorlands.jspzQzpgzEzzSzppdocszSzuszSzcommonzSzdorlandszSzdorlandzSzdmd_n_10zPzhtm[16] Lanni C, Lenzken SC, Pascale A et al. . (March 2008). "Cognition enhancers between treating and doping the mind". Pharmacol. Res. 57 (3): 196-213.doi:10.1016/j.phrs.2008.02.004. PMID 18353672.[17] Giurgea C (1972). "[Pharmacology of the integrative activity of the brain. An attempt at a nootropic concept in psychopharmacology][18] BBC online investigation into Modafinil. Available at - http://www.dailymail.co.uk/health/article- 2451586/More-students-turn-the-cognitive-enhancing-drug-Modafinil-hope-boosting-grades-job-perspects.html