[13] Hydroxyzine can also be used for the treatment of allergic conditions, such as chronic urticaria, atopic or contact dermatoses, and histamine-mediated pruritus.

[medical citation needed] These have also been confirmed in both recent and past studies to have no adverse effects on the liver, blood, nervous system, or urinary tract.

When administered to pregnant rats, mice, and rabbits, hydroxyzine caused abnormalities such as hypogonadism with doses significantly above that of the human therapeutic range.

[22][better source needed] In humans, a significant dose has not yet been established in studies, and, by default, the US Food and Drug Administration (FDA) has introduced contraindication guidelines regarding hydroxyzine.

[22] Other contraindications include the administration of hydroxyzine alongside depressants and other compounds that affect the central nervous system;[22] if necessary, it should only be administered concomitantly in small doses.

[22] If administered in small doses with other substances, as mentioned, then patients should refrain from using dangerous machinery, motor vehicles, or any other practice requiring absolute concentration, under safety laws.

[22] Studies have also been conducted which show that long-term prescription of hydroxyzine can lead to tardive dyskinesia after years of use, but effects related to dyskinesia have also anecdotally been reported after periods of 7.5 months,[23] such as continual head rolling, lip licking, and other forms of athetoid movement.

In certain cases, elderly patients' previous interactions with phenothiazine derivatives or pre-existing neuroleptic treatment may have contributed to dyskinesia at the administration of hydroxyzine due to hypersensitivity caused by prolonged treatment,[23] and therefore some contraindication is given for short-term administration of hydroxyzine to those with previous phenothiazine use.

[23] Several reactions have been noted in manufacturer guidelines—deep sleep, incoordination, sedation, and dizziness have been reported in children and adults, as well as others such as hypotension, tinnitus, and headaches.

[24][medical citation needed] Central nervous system effects such as hallucinations or confusion have been observed in rare cases, attributed mostly to overdosage.

The hallucinogenic or hypnotic properties have been described as being an additional effect from overall central nervous system suppression by other CNS agents, such as lithium or ethanol.

[29] Hydroxyzine's predominant mechanism of action is as a potent and selective histamine H1 receptor inverse agonist.

[31][39] Similarly to the atypical antipsychotics, the comparably weak antiserotonergic effects of hydroxyzine likely underlie its usefulness as an anxiolytic.

[39] A positron emission tomography (PET) study found that brain occupancy of the H1 receptor was 67.6% for a single 30 mg dose of hydroxyzine.

[7] One study found that the elimination half-life of hydroxyzine in adults was as short as 3 hours, but this may have just been due to methodological limitations.