The present invention relates to 1-(1-methylethyl)-4-{[4-(tetrahydro-2H-pyran-4-yloxy)phenyl]carbonyl}piperazine or a pharmaceutically acceptable salt thereof, in particular the hydrochloride salt thereof and crystalline forms of the hydrochloride salt; to processes for the preparation of the compound or its salt; to compositions containing it; and to its use in the treatment or prophylaxis of neurological or psychiatric diseases, such as cognitive impairment, fatigue or a sleep disorder, for example in a mammal such as a human. The compound or a salt thereof has affinity for and is an antagonist and/or inverse agonist of the histamine H3 receptor.