Antihistamine
The vast majority of marketed H-antihistamines are receptor antagonists and only a minority of marketed compounds are inverse agonists at the receptor. Clinically, H-antihistamines are used to treat allergic reactions and mast cell-related disorders. Sedation is a common side effect of H-antihistamines that readily cross the blood–brain barrier; some of these drugs, such as diphenhydramine and doxylamine, are therefore used to treat insomnia. H-antihistamines can also reduce inflammation, since the expression of NF-κB, the transcription factor the regulates inflammatory processes, is promoted by both the receptor's constitutive activity and agonist (i.e., histamine) binding at the H receptor.
Recreational use of dextromethorphan
A formal survey of dextromethorphan users showed that more than half of users reported experience of the following withdrawal-symptoms individually for the first week after long-term/addictive dextromethorphan use: fatigue, apathy, flashbacks, and constipation. Over a quarter reported insomnia, nightmares, anhedonia, impaired memory, attention deficit and decreased libido. Rarer side effects included panic attacks, impaired learning, tremor, jaundice, urticaria (hives) and myalgia. DXM has also been "known to increase the frequency of complex partial seizures in epileptics by 25% compared to placebo." Frequent and long-term usage at high doses could possibly lead to toxic psychosis and other permanent psychological problems. It is important to note that medical DXM use has not been shown to cause the above issues.