thumb|right|250px|An assortment of psychoactive drugs

A '''psychoactive drug''' or '''psychotropic substance''' is a mood, recreationally to purposefully alter one's consciousness, as entheogens for ritual or spiritual purposes, as a tool for studying or augmenting the mind, or therapeutically as medication.

Because psychoactive substances bring about subjective changes in consciousness and mood that the user may find pleasant (e.g. euphoria) or advantageous (e.g. increased alertness), many psychoactive substances are abused, that is, used excessively, despite risks or negative consequences. With sustained use of some substances, physical dependence may develop, making the cycle of abuse even more difficult to interrupt. Drug rehabilitation can involve a combination of psychotherapy, support groups and even other psychoactive substances to break the cycle of dependency.

In part because of this potential for abuse and dependency, the ethics of drug use are the subject of a continuing philosophical debate. Many governments worldwide have placed restrictions on drug production and sales in an attempt to decrease drug abuse.

== History ==

Drug use is a practice that dates to prehistoric times. There is archaeological evidence of the use of psychoactive substances dating back at least 10,000 years, and historical evidence of cultural use over the past 5,000 years.<ref name="merlin"></ref> While medicinal use seems to have played a very large role, it has been suggested that the urge to alter one's consciousness is as primary as the drive to satiate thirst, hunger or sexual desire.<ref name="siegel"></ref> Others suggest that marketing, availability or the pressures of modern life are why humans use so many psychoactives in their daily lives. However, the long history of drug use and even children's desire for spinning, swinging, or sliding indicates that the drive to alter one's state of mind is universal.<ref name="weil"></ref>

This relationship is not limited to humans. A number of animals consume different psychoactive plants, animals, berries and even fermented fruit, becoming intoxicated, such as cats after consuming catnip. Traditional legends of sacred plants often contain references to animals that introduced humankind to their use.<ref name="samorini"></ref> Biology suggests an evolutionary connection between psychoactive plants and animals, as to why these chemicals and their receptors exist within the nervous system.<ref name="albert"></ref>

The 20th century has seen governments initially responding to many drugs by banning them and making their use, supply or trade a criminal offense. A notable example of this is the Prohibition era in the United States, where alcohol was made illegal for 13 years. However, many governments have concluded that illicit drug use cannot be sufficiently stopped through criminalization. In some countries, there has been a move toward harm reduction by health services, where the use of illicit drugs is neither condoned nor promoted, but services and support are provided to ensure users have the negative effects of their illicit drug use minimized. This can go hand-in-hand with supply reduction strategies by law-enforcement agencies.

== Uses of psychoactive substances ==

Psychoactive substances are used by humans for a number of different purposes, both legal and illicit.  

=== Anesthesia ===

General anesthetics are a class of psychoactive drug used on patients to block pain and other sensations. Most anesthetics induce unconsciousness, which allows patients to undergo medical procedures like surgery without physical pain or emotional trauma.<ref>Medline Plus. [http://www.nlm.nih.gov/medlineplus/anesthesia.html Anesthesia.] Accessed on July 16, 2007.</ref> To induce unconsciousness, anesthetics affect the GABA and NMDA systems. For example, halothane is a GABA agonist,<ref></ref> and ketamine is an NMDA receptor antagonist.<ref></ref>

=== Painkillers ===

thumb|right|135px|Aspirin
Psychoactive drugs are often prescribed to manage pain. As the subjective experience of pain is regulated by endorphins, neurochemicals that are endogenous opioids, pain can be managed using psychoactives that operate on this neurotransmitter system. This class of drugs includes narcotics, like morphine and codeine,<ref></ref> and also NSAIDs such as aspirin and ibuprofen.

=== Psychiatric medications ===
 
 
thumb|right|Zoloft, an [[antidepressant  (and anti-anxiety) medication]]
Psychiatric medications are prescribed for the management of mental and emotional disorders. There are 6 major classes of psychiatric medications: 
*Antidepressants, which are used to treat disparate disorders such as clinical depression, dysthymia, anxiety, eating disorders and borderline personality disorder.<ref>
{{cite journal
 | last = Schatzberg
 | first = A.F.
 | year = 2000
 | title = New indications for antidepressants.
 | journal = Journal of Clinical Psychiatry
 | volume = 61
 | issue = 11
 | pages = 9–17
 | pmid = 10926050 
 | accessdate = 2007-11-20
}}
</ref>
*suppress the appetite. 
*Antipsychotics, which are used to treat psychoses such as schizophrenia and mania.
*Mood stabilizers, which are used to treat bipolar disorder and schizoaffective disorder.
*Anxiolytics, which are used to treat anxiety disorders.
*Depressants, which are used as hypnotics, sedatives, and anesthetics.

=== Recreational drugs ===

Many psychoactive substances are used for their mood and perception altering effects, including those with accepted uses in medicine and psychiatry. Classes of drugs frequently used recreationally include:
*euphoric and performance-enhancing effects.
*Hallucinogens, which induce perceptual and cognitive distortions. 
*Hypnotics, which are used recreationally because they induce inebriation.
*euphoric effects.
*Inhalants, in the forms of gas aerosols, or solvents, which are inhaled as a vapor inducing stupefying effects.
Examples include alcohol, cocaine, LSD, and cannabis.<ref name="who">[http://www.who.int/substance_abuse/publications/en/Neuroscience_E.pdf Neuroscience of Psychoactive Substance Use and Dependence] by the WHO. Retrieved on July 5, 2007.</ref>

In many cultures, possessing or having used recreational drugs is seen as a status symbol. Recreational drugs, especially those known as club drugs, are seen as status symbols at social events such as at nightclubs and parties.<ref></ref> This is true of many cultures throughout history; drugs have been viewed as status symbols since ancient times. For example, in ancient Egypt, gods were commonly pictured holding hallucinogenic plants.<ref></ref>

=== Ritual and spiritual use ===

thumb|right|150px|[[Timothy Leary was a leading proponent of spiritual hallucinogen use.]]

Certain psychoactives, particularly hallucinogens, have been used for religious purposes since prehistoric times. Native Americans have used jimsonweed, cannabis have been used in religious ceremonies for centuries.<ref name="hall">Hall, Andy. [http://www.nerdshit.com/wordpress/?p=1433 Entheogens and the Origins of Religion]. Retrieved on May 13, 2007.</ref> 

The use of entheogens for religious purposes resurfaced in the West during the counterculture movements of the 1960s and 70s. Under the leadership of Timothy Leary, new religious movements began to use LSD and other hallucinogens as sacraments.<ref></ref> In the United States, the use of peyote for ritual purposes is protected only for members of the Native American Church, which is allowed to cultivate and distribute peyote. However, the bona fide religious use of Peyote, regardless of ancestry, is protected in Colorado, Arizona, New Mexico, Nevada, and Oregon.<ref></ref>

== Administration ==

For a substance to be psychoactive, it must cross the administered in several different ways. In medicine, most psychiatric drugs, such as capsule form. However, certain medical psychoactives are administered via injection, or rectal alcohol and smoked; insufflated. The efficiency of each method of administration varies from drug to drug.<ref>United States Food and Drug Administration. [http://www.fda.gov/cder/dsm/DRG/drg00301.htm CDER Data Standards Manual]. Retrieved on May 15, 2007.</ref>

'''向精神薬'''（こうせいしんやく、Psychoactive drug）は、広義には、中枢神経系に作用し、生物の精神活動に何らかの影響を与える薬物の総称である。狭義には、麻薬及び向精神薬取締法で個別に指定された物質である。以下では広義の向精神薬について述べる。

脳内で本来はたらいている神経伝達物質をリガンド、リガンドのかわりに脳内で作用する薬物をアゴニスト、リガンドのはたらきを弱める薬物をアンタゴニストという。

精神治療薬の中に副作用として幻覚や錯乱などの影響をおこすものがあり、反対に、麻薬に指定された薬物の中にも、覚醒作用や鎮痛・鎮静作用などの作用を持つために治療に用いられるものが存在する。例えば、モルヒネは強い依存性を持つが、同時に強力な鎮痛作用が存在するため、重度の癌患者の疼痛除去に用いられている。また、メタンフェタミンのように、日本では覚せい剤取締法で法律で規制されているが、北アメリカでは治療薬として用いられ、規制の状況が異なる薬物もある。

また、医療目的で用いられる治療薬も法律で規制された薬物も、共に薬物乱用が問題になることがある。

== 精神治療薬 ==
精神疾患によっては、脳の中に薬物が作用し、その症状を顕著に改善する。ストレスや遺伝などの様々な原因によって脳内の神経伝達物質の状況が変化するため、その類似の物質である薬物を摂取することによって症状が改善されるということである。反対に薬物が効きにくいといわれる疾患には拒食症、高所恐怖症、閉所恐怖症などがある<ref>石浦章一 『遺伝子が明かす脳と心のからくり-東京大学超人気講義録』 羊土社、2004、ISBN 978-4897068824。36、71頁。</ref>。

軽い不安は脳内物質のセロトニン、強い不安は脳内物質のGABAの不足が関連しており<ref>石浦章一 『遺伝子が明かす脳と心のからくり-東京大学超人気講義録』 羊土社、2004、ISBN 978-4897068824。81頁。</ref>、脳内物質として作用する薬物を摂取することで症状を抑える。ベンゾジアゼピン系の薬物はGABA受容体に作用することで不安や興奮を抑える。強迫性障害のような持続的な不安には、GABA受容体ではなく、セロトニン受容体に作用する抗うつ薬が有効である。

=== 治療薬の種類 ===

; 精神安定剤（トランキライザー）: 大きくメジャートランキライザーとマイナートランキライザーに分けられる。
:;抗精神病薬（メジャートランキライザー）: 主に統合失調症による幻覚や妄想といった症状を抑えるために処方される。また強力な睡眠薬であるベゲタミンもここに分類される。統合失調症に有効な抗精神病薬は、ドーパミンD2受容体に作用し脳内物質のドーパミンの作用を抑える、つまりドーパミンのアンタゴニストである。
:;抗不安薬（マイナートランキライザー） : 不安や緊張を沈める作用があり、睡眠薬としても利用される。ベンゾジアゼピン系が多い。
; 抗うつ薬 : うつ病や強迫性障害、社会不安障害に処方される。主要な作用は、セロトニンが少なくなっている状態に対してセロトニンを再利用する作用をもたらすことである。第1世代・第2世代の抗うつ薬である三環系抗うつ薬、第2世代の四環系抗うつ薬、第3世代の選択的セロトニン再取り込み阻害薬(SSRI)、第4世代のセロトニン・ノルアドレナリン再取り込み阻害薬(SNRI)がある。新しいモノアミン酸化酵素阻害薬(MAOI)であるRIMA(Reversible inhibitors of monoamine oxidase type-A)という新薬もある。

; 抗躁薬 : 気分が高まった躁状態を改善するための治療薬である。
; 中枢神経刺激薬 : メチルフェニデート（リタリン）やアンフェタミンのように、突然強い眠気を催すナルコレプシーや注意欠陥・多動性障害(ADHD)の治療薬として処方される。メチルフェニデートやアンフェタミンは、ドーパミンの受容体に結合する。
; 睡眠導入剤 : 不眠症に対し、睡眠を誘導する治療薬として用いられる。 ベンゾジアゼピン系が多い。
; 鎮静催眠薬 : バルビタールなど、強い催眠作用のある薬物で、睡眠薬として用いられた。
; 抗ヒスタミン薬

以上のうち、主に抗不安薬、中枢神経刺激薬、鎮静催眠薬の一部が麻薬及び向精神薬取締法で向精神薬に指定されている。