An anaphrodisiac or antiaphrodisiac is something that quells or blunts the libido. It is the opposite of an aphrodisiac, something that enhances sexual appetite. The word anaphrodisiac comes from the Greek prefix αν-, denoting negation, and the Greek goddess of love, Aphrodite.
Available anaphrodisiacs classes of substances 
There are two broad sources of pharmaceutical anaphrodisiac products available for those wishing to decrease sex drive: herbal and synthetic.
Some common anaphrodisiacs are alcohol and tobacco, but this is typically an unintended consequence and not the main reason for use. While alcohol is used socially because it initially reduces mental inhibitions, studies have shown that over time alcohol physically decreases arousal and makes achieving climax more difficult. For this reason alcohol is considered an anaphrodisiac.
Less natural options are prescription synthetic compounds such as estrogens and anti-androgen drugs. These drugs have several side effects including fatigue, headaches, mood disturbances, loss of body hair, and breast development. Because of the health risks of prescription medication, these products are undesirable for a large segment of the population and are most frequently prescribed by professionals for male habitual sex offenders. These products are considered too risky for the general public and are typically not recommended for casual use.
Side effects of certain antidepressant medications may be anaphroditic in nature (e.g. SSRIs and certain antipsychotics), however, there are no prescription psychotropic drugs that have reliable anaphroditic effects..
Antiandrogen drugs such as cyproterone or medroxyprogesterone are sometimes prescribed to convicted sex offenders such as rapists and pedophiles who are released on parole in an effort to stop them reoffending, however the high doses required often cause a range of side effects which may limit compliance.
Reasons given for people to take anaphrodisiacs 
There are several commonly cited uses for anaphrodisiacs. People with an important presentation, exam or social event may be interested in suppressing distracting sexual urges that detract from mental focus. Similar intrusive thoughts may also waste emotional energy during dating. Physical response, especially penile erection, to sexual thoughts at inappropriate times may cause social embarrassment.
Herbal anaphrodisiacs have been employed by various religious sects and orders throughout history. Most commonly, Chaste Tree (Vitex agnus-castus) has been used to normalize hormones in both men and women. An over-active libido is very often treated herbally by addressing poor adrenal function.
Rumours that the British Army put the 19th century anticonvulsant and sedative potassium bromide in soldiers' tea during World War II to damp soldiers' lust appears to be an urban myth. Given the long half-life of the drug in the body, a mildly sedated army would be unlikely to be an effective fighting force. A similar belief appears to exist in the United States about saltpeter in army coffee or in Russia about potassium bromide in army food.
Barrister Sir Edward Marshall Hall theorised that murderer Hawley Harvey Crippen was using hyoscine on his wife as an anaphrodisiac but accidentally gave her an overdose and then panicked when she died.
Scientific study 
Studies have evaluated the effect of herbal anaphrodisiacs on men and women. These include studies on the effect of substances on both hormone levels and behavior.
The mechanism of the active component of some plant-based anaphrodisiacs may be the induction of enzymes that catalyze the conversion of sex-hormone precursors into androstenedione, which promotes the reduction of sexual urges. Studies have demonstrated that some of these products inhibit 17ß-hydroxysteroid dehydrogenase and 17,20-lyase, which catalyzes the conversion of 17-hydroxyprogesterone of androstenedione to testosterone.
In one study a group of men were given an anaphrodisiac and the effect on the metabolism of mineralocorticoids in these men was recorded. During the period of administration, the testosterone concentrations decreased and the serum 17-hydroxyprogesterone concentrations increased. Testosterone levels fell by about 40% after administration and returned to normal after usage was discontinued. However, the actual effect of these medicines on sexual desire was not measured in the study.
The amino acid 5-HTP, or 5-hydroxytryptophan, has been anecdotally reported to be a mild anaphrodisiac, as has the serotonergic empathogen MDMA, popularly known as "ecstasy". However, systematic study of these chemicals is lacking, due to the off-patent nature of 5-HTP, and the legal control of MDMA. In addition, other serotonergic euphoriant drugs, like the psychedelic LSD, have been reportedly used to drastically increase sexual pleasure. MDMA in combination with a PDE5 inhibitor (trade names Viagra, Levitra, and Cialis) is termed "sextasy". This combination increases libido and performance simultaneously. A PDE5 inhibitor taken alone has no effect on sex drive, suggesting that serotonergic euphoriants like MDMA may actually increase libido while decreasing performance, similar to another common drug: drinking alcohol.
In 2012 Iranian researchers of the Shahid Beheshti University of Medical Sciences in Tehran have found that liquorice - especially used in sweets, tea and herbal remedies - can lower levels of testosterone. Glycyrrhiza, the compound in liquorice root contains phytoestrogens and has been shown to affect the endocrine system. The findings backs up a previous Italian study about the hormonal effects of liquorice whose results could not be replicated in other studies by University of Texas and University of Michigan in the past. The Iranian scientists came to the conclusion that the regular consumption of liquorice can lower libido of men and in high doses may even increase the risks of sexual problems including impotence. However, the study also showed that the level of testosterone normalizes after abstinence of liquorice within a few days and the effects completely disappear. For women, the consumption of liquorice has an opposite effect on their sexual desire and can improve female libido.
See also 
List of putative anaphrodisiacs 
- Heracleum (plant)
- Wild lettuce
- Common Rue
- New Scientist: "The Last Word", 29 June 2002[dead link] debunks bromide in British Army tea
- snopes.com debunks saltpetre in U.S. Army coffee
- Aphrodisiacs and Anti-aphrodisiacs: Three Essays on the Powers of Reproduction by John Davenport.
- Avoid the Sex by Feeding Them Anaphrodisiacs
- Saltpeter in Food
- Browne, Douglas G.; Tullett, E.V. (1955). Bernard Spilsbury: his life and cases. Harmondsworth: Penguin Books. pp. 31–54.
- Filson Young (1954). In Harry Hodge. Famous Trials I. Harmondsworth: Penguin Books. p. 124.
- Farese RV Jr, Biglieri EG, Shackleton CHL, Irony I, Gomez-Fontes R. glycyrrhetinic acid-induced hypermineralocorticoidism. N Engl J Med 1991;325:1223-1227. [Medline]
- Yaginuma T, Izumi R, Yasui H, Arai T, Kawabata M Effect of traditional herbal medicine on serum testosterone levels and its induction of regular ovulation in hyperandrogenic and oligomenorrheic women [Article in Japanese]. Nippon Sanka Fujinka Gakkai Zasshi 1982 Jul;34(7):939-44
- "Liquorice risk to men's sex life". BBC News. 17 September 2003.
- "Hyrtl mentions (loc. cit. ii, p. 94), rue (Ruta graveolens) was considered a sexual sedative by the monks of old, who on this account assiduously cultivated it in their cloister gardens to make vinum rutæ."