The following substances are known to be HNMT inhibitors: amodiaquine, chloroquine, dimaprit, etoprine, metoprine, quinacrine, SKF-91488, tacrine, and diphenhydramine. HNMT inhibitors may increase histamine levels in peripheral tissues and aggravate conditions associated with histamine excess, such as allergic rhinitis, urticaria, and peptic ulcer disease.  the effect of HNMT inhibitors on brain function is not yet fully understood. Research suggests that using new inhibitors of HNMT to increase the levels of histamine in the brain could potentially contribute to improvements in the treatment of brain disorders.