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.
Physostigmine
It has been shown to improve long term memory, and was once explored as a therapy for Alzheimer's disease, but in clinical trials it was not shown to confer convincing benefits, and it led to very common moderate to severe side-effects such as nausea, vomiting, diarrhea, loss of appetite (anorexia), abdominal pain, and tremors, resulting in a high rate of withdrawal. Physostigmine's poor tolerability led to it being abandoned in favor of later acetylcholinesterase inhibitors, three of which are currently in use: donepezil, galantamine, and rivastigmine. Recently, it has begun to be used in the treatment of orthostatic hypotension.