Lysergide (LSD) is a semi-synthetic hallucinogen and is one of the most potent drugs known. Recreational use became popular between the 1960s to 1980s, but is now less common. LSD was first synthesized by Albert Hoffman while working for Sandoz Laboratories in Basel in 1938. Some years later, during a re-evaluation of the compound, he accidentally ingested a small amount and described the first ‘trip’. During the 1950s and 1960s, Sandoz evaluated the drug for therapeutic purposes and marketed it under the name Delysid®. It was used for research into the chemical origins of mental illness. Recreational use started in the 1960s and is associated with the ‘psychedelic period’. LSD possesses a complex pharmacological profile that includes direct activation of serotonin, dopamine and norepinephrine receptors. In addition, one of its chief sites of action is that of compound-specific (“allosteric”) alterations in secondary messengers associated with 5HT2A and 5HT2C receptor activation and changes in gene expression. The hallucinogenic effects of LSD are likely due to agonism at 5HT2A and 5HT2C receptors. LSD is also an agonist at the majority of known serotonin receptors, including 5HT1A, 5HT1B, 5HT1D, 5HT5A, 5HT6 and 5HT7 receptors. During the 1960s, LSD was investigated for a variety of psychiatric indications, including the following: as an aid in treatment of schizophrenia; as a means of creating a "model psychosis"; as a direct antidepressant; and as an adjunct to psychotherapy. LSD is listed in Schedule I of the United Nations 1971 Convention on Psychotropic Substances.

PNU-96391A (known as OSU6162) is a weak dopamine (DA) D(2) receptor antagonist with behavioral stabilizing properties. OSU6162 seem to act as stabilizers not only on dopaminergic, but also on serotonergic brain signaling (partial agonist on 5-HT2A receptor). OSU6162 in a phase II European clinical trial in treatment of Myalgic Encephalomyelitis/Chronic Fatigue Syndrome. One of the isomer of OSU 6162, has promise for treating Parkinson's disease, Huntington's disease and schizophrenia, but both enantiomers of OSU 6162 had dual effects on behavior, stimulating locomotor activity in 'low activity' animals and inhibiting locomotor activity in 'high activity' animals.

Lerimazoline is a sympathomimetic drug that belongs to the imidazoline class of compounds, and is used as a nasal decongestant. Lerimazoline displayed high affinity for the 5-HT1A receptor and for the 5-HT1D receptor. Binding affinity estimates for α1-adrenoceptor, 5-HT2A, and D2 receptors were more than ten times lower. The mechanism of vasoconstrictor action of lerimazoline encompasses both, the activation of 5-HT2A, and to a lesser degree α1 -adrenergic receptors. These results also suggest that lerimazoline is an “atypical” decongestant. It inhibits secretion of nasal mucus. Lerimazoline causes hypertension.

CYR-101 (MIN-101) is a cyclic amide derivative that has high equipotent affinities for 5-HT2A and sigma-2 receptors (Ki of 7.53 nM and 8.19 nM for 5-HT2A and sigma-2, respectively). MIN-101 also shows binding affinity for a1-adrenergic receptors but low or no affinity for muscarinic, cholinergic, and histaminergic receptors. MIN-101 demonstrated statistically significant efficacy in reducing negative symptoms and good tolerability in stable schizophrenia patients. The drug is in phase II clinical trials for the treatment of Schizophrenia.

Temanogrel (also known as APD791) is an oral small molecule inverse agonist of the serotonin 2A (5-HT2A) receptor with potent activity on platelets and vascular smooth muscle. Temanogrel has been studied in phase I clinical trials in healthy subjects to assess its pharmacokinetics, pharmacodynamics, and safety. However, these studies were terminated because of the sponsor’s decision.

Volinanserin (MDL-100,907) is a highly selective 5-HT2A receptor antagonist. It is widely used in scientific research to investigate the function of the 5-HT2A receptor. Volinanserin is also being trialed as a potential antipsychotic, antidepressant and treatment for insomnia. Volinanserin (M-100907) was in phase III trials for chronic schizophrenia. In August 1999, development was discontinued for acute schizophrenia (schizoaffective disorder) on the basis of poor results. M-100907 is also active in animal models involving blockade of NMDA glutamatergic channel receptors, an effect known to resemble some behavioral symptoms of schizophrenia in man. M-100907 is also claimed in other patents for the treatment of thromboembolic disorders, for the treatment of various developmental neurological disorders such as autism and attention deficit hyperactivity disorder.

Pruvanserin (EMD 281014, LY-2422347) is a selective serotonin 5-HT2A receptor antagonist. Pruvanserin was originated by Merck KGaA. Eli Lilly had been developing pruvanserin, under a global licence from Merck KGaA, for the treatment of primary insomnia and major depressive disorder. Phase II trials were completed in the US, Hungary and Spain. However, development appears to have been discontinued.

Pirenperone, a quinazoline derivative, is a selective antagonist at serotonin receptor 2A binding sites. The liposoluble compound pirenperone has been studied in a variety of behavioral tests including the sensitive d-lysergic acid diethylamide (LSD) cue discrimination assay, in which it served as a potent LSD-antagonist. Pirenperone also proved to be an effective antagonist of serotonin-mediated behavioral responses including the head twitch response thought to be mediated by serotonin receptors.

Biriperone (also known as Centbutindole) was developed as an antipsychotic drug. It has shown effective antipsychotic activity in schizophrenic patients. Biriperone acts as a dopamine antagonist, and also as a blocker of 5HT(2) receptor. Clinically the drug has passed through phase I, II & III clinical trials successfully, however, the use of this drug was banned.