Are antihistamines considered to be psychoactive substances?
Short answer
First-generation anti-histamines are psychoactive, second-generation equivalents may not be*.

Background
According to WHO, psychoactive substances are: 


  [...] substances that, when taken in or administered into one's system, affect mental processes, e.g. cognition or affect. This term and its equivalent, psychotropic drug, are the most neutral and descriptive term for the whole class of substances, licit and illicit, of interest to drug policy. ‘Psychoactive’ does not necessarily imply dependence-producing, and in common parlance, the term is often left unstated, as in ‘drug use’ or ‘substance abuse’.


Antihistamines are used to treat allergies. First-generation antihistamines readily cross the blood-brain-barrier and have been associated with performance deficits on tests of attention, working memory, vigilance, and speed. By contrast, second-generation antihistamines that do not readily reach the brain do not have these side-effects (Kay, 2000). 

Since attention, working memory and vigilance are mental cognitive processes, first-generation antihistamines fall into the classification of WHO as being psychoactive.

Reference
- Kay, J Allergy Clin Immunol (2000); 105(6-2): S622–7

Note
*I am not an MD and data on 2nd generation drugs are based on a single study with a limited number of tests. Consult a medical professional for prescription of the appropriate treatment.