Conessine is a steroidal alkaloid found in a number of plant species from the family Apocynaceae, including Holarrhena floribunda, Holarrhena antidysenterica and Funtumia elastica. It acts as a histamine antagonist, selective for the H<sub>3</sub> subtype (with an affinity of pK<sub>i</sub> = 8.27; K<sub>i</sub> = ~5 nM). It was also found to have long CNS clearance times, high bloodâÂÂbrain barrier penetration and high affinity for the adrenergic receptors.