[The secondary effects of the antihistamine chlorpheniramine on the central nervous system]

The objective of this study is to review the main investigations into the secondary effects of the antihistamine chlorpheniramine on the central nervous system (CNS).

The antagonists of the H1 receptors of histamine, usually used in the treatment of symptoms of allergy or the common cold, have many adverse effects on the CNS. They cause day time drowsiness, cause poorer performance of tasks involving visuo motor coordination and make it more difficult to detect target auditory stimuli in tasks involving sustained concentration. When using evoked potentials (EP) it has been observed that they alter the system for maintaining auditory attention. They cause increased P300 latency, an EP related to the voluntary ability to discriminate between relevant stimuli, a reduction in the amplitude of mismatch negativity (MMN), an EP which is seen as a pre attention mechanism for automatic detection of environmental acoustic changes and alters selective attention capacity, reflected by a reduction in the amplitude of processing negativity (PN).

These studies show that chlorpheniramine has major adverse effects on the CNS, and the patient may not be subjectively aware of this (e.g. selective attention). This means that in certain situations it is a dangerous substance. The characteristics of these adverse effects should lead to a review of the prescription of chlorpheniramine, and stimulate the search for other substances with similar therapeutic actions but fewer side effects on the CNS.