Illustration of the major elements of neurotransmission. Depending on its method of action, a<unk>  substance may block the receptors on the post-synaptic neuron (dendrite), or block reuptake or affect neurotransmitter synthesis in the pre-synaptic neuron (axon).<unk> s operate by temporarily affecting a person's neurochemistry, which in turn causes changes in a person's mood, cognition, perception and behavior. There are many ways in which<unk> s can affect the brain. Each drug has a specific action on one or more neurotransmitter or neuroreceptor in the brain. Drugs that increase activity in particular neurotransmitter systems are called agonists. They act by increasing the synthesis of one or more neurotransmitters, by reducing its reuptake from the synapses, or by mimicking the action by binding directly to the postsynaptic receptor. Drugs that reduce neurotransmitter activity are called antagonists, and operate by interfering with synthesis or blocking postsynaptic receptors so that neurotransmitters cannot bind to them. Exposure to a<unk>  substance can cause changes in the structure and functioning of neurons, as the nervous system tries to re-establish the homeostasis disrupted by the presence of the drug (see also, neuroplasticity). Exposure to antagonists for a particular neurotransmitter can increase the number of receptors for that neurotransmitter or the receptors themselves may become more responsive to neurotransmitters; this is called sensitization. Conversely, overstimulation of receptors for a particular neurotransmitter may cause a decrease in both number and sensitivity of these receptors, a process called desensitization or tolerance. Sensitization and desensitization are more likely to occur with long-term exposure, although they may occur after only a single exposure. These processes are thought to play a role in drug dependence and addiction. Physical dependence on antidepressants or anxiolytics may result in worse depression or anxiety, respectively, as withdrawal symptoms. Unfortunately, because clinical depression (also called major depressive disorder) is often referred to simply as depression, antidepressants are often requested by and prescribed for patients who are depressed, but not clinically depressed. Affected neurotransmitter systems The following is a brief table of notable drugs and their primary neurotransmitter, receptor or method of action. Many drugs act on more than one transmitter or receptor in the brain. Neurotransmitter/receptor Classification Examples 150px Cholinergics (acetylcholine receptor agonists) arecoline, nicotine, piracetam Muscarinic antagonists (acetylcholine receptor antagonists) scopolamine, benzatropine, dimenhydrinate, diphenhydramine, doxylamine, atropine, quetiapine, olanzapine, most tricyclics Nicotinic antagonists (acetylcholine receptor antagonists) memantine, bupropion 150px Adenosine receptor antagonists caffeine, theobromine, theophylline 150px Dopamine reuptake inhibitors cocaine, bupropion, methylphenidate, St John's wort, and certain TAAR1 agonists like amphetamine, phenethylamine, and methamphetamine Dopamine releasing agents Cavendish bananas, TAAR1 agonists like amphetamine, phenethylamine, and methamphetamine Dopamine agonists pramipexole, Ropinirole, L-DOPA (prodrug), memantine Dopamine antagonists haloperidol, droperidol, many antipsychotics (e.g., risperidone, olanzapine, quetiapine) Dopamine partial agonists LSD, aripiprazole 150px GABA reuptake inhibitors tiagabine, St John's wort, vigabatrin, deramciclane GABAA receptor agonists ethanol, niacin, barbiturates, diazepam, clonazepam, lorazepam, temazepam, alprazolam and other benzodiazepines, zolpidem, eszopiclone, zaleplon and other nonbenzodiazepines, muscimol, phenibut GABAA receptor positive allosteric modulators GABA receptor antagonists thujone, bicuculline GABAA receptor negative allosteric modulators 150px Norepinephrine reuptake inhibitors St John's wort, most non-SSRI antidepressants such as amoxapine, atomoxetine, bupropion, venlafaxine, quetiapine, the tricyclics, methylphenidate, SNRIs such as duloxetine, venlafaxine, cocaine, tramadol, and certain TAAR1 agonists like amphetamine, phenethylamine, methamphetamine. Norepinephrine releasing agents ephedrine, PPA, pseudoephedrine, amphetamine, phenethylamine, methamphetamine Adrenergic agonists clonidine, guanfacine, phenylephrine Adrenergic antagonists carvedilol, metoprolol, mianserin, prazosin, propranolol, trazodone, yohimbine, olanzapine 150px Serotonin receptor agonists triptans (e.g. sumatriptan, eletriptan), psychedelics (e.g. lysergic acid diethylamide, psilocybin, mescaline), ergolines (e.g. lisuride, bromocriptine) Serotonin reuptake inhibitors most antidepressants including St John's wort, tricyclics such as imipramine, SSRIs (e.g. fluoxetine, sertraline, escitalopram), SNRIs (e.g. duloxetine, venlafaxine) Serotonin releasing agents fenfluramine, MDMA (ecstasy), tryptamine Serotonin receptor antagonists ritanserin, mirtazapine, mianserin, trazodone, cyproheptadine, memantine, atypical antipsychotics (e.g., risperidone, olanzapine, quetiapine) 150px AMPA receptor positive allosteric modulators aniracetam, CX717, piracetam AMPA receptor antagonists kynurenic acid, NBQX, topiramate 150px Cannabinoid receptor agonists JWH-018 Cannabinoid receptor partial agonists Anandamide, THC, cannabidiol, cannabinol Cannabinoid receptor inverse agonists Rimonabant Anandamide reuptake inhibitors LY 2183240, VDM 11, AM 404 FAAH enzyme inhibitors MAFP, URB597, N-Arachidonylglycine NMDA receptor antagonists ethanol, ketamine, deschloroketamine, 2-Fluorodeschloroketamine, PCP, DXM, Nitrous Oxide, memantine GHB receptor agonists GHB, T-HCA Sigma receptor Sigma-1 receptor agonists cocaine, DMT, DXM, fluvoxamine, ibogaine, opipramol, PCP, methamphetamine Sigma-2 receptor agonists methamphetamine Opioid receptor μ-opioid receptor agonists Narcotic opioids (e.g. codeine, morphine, hydrocodone, hydromorphone, oxycodone, oxymorphone, heroin, fentanyl) μ-opioid receptor partial agonists buprenorphine μ-opioid receptor inverse agonists naloxone μ-opioid receptor antagonists naltrexone κ-opioid receptor agonists salvinorin A, butorphanol, nalbuphine, pentazocine, ibogaine κ-opioid receptor antagonists buprenorphine H1 receptor antagonists diphenhydramine, doxylamine, mirtazapine, mianserin, quetiapine, olanzapine, meclozine, most tricyclics H3 receptor antagonists pitolisant Monoamine oxidase inhibitors (MAOIs) phenelzine, iproniazid, tranylcypromine, selegiline, rasagiline, moclobemide, isocarboxazid, Linezolid, benmoxin, St John's wort, coffee, garlic Melatonin receptor agonists agomelatine, melatonin, ramelteon, tasimelteon Imidazoline receptor agonists apraclonidine, clonidine, moxonidine, rilmenidine Orexin receptor Orexin receptor agonists modafinil Orexin receptor antagonists SB-334,867, SB-408,124, TCS-OX2-29, suvorexant
Psychoactive drug
Heroic medicine
Antiprion drug
Antimanic drugs
Topological drugs
Psychedelic drug
Alcohol 
Botanical drug