Fenarvet

Antimuscarinic effects (atropine-like effects) are common. Do not use in conditions for which anticholinergic drugs may be contraindicated, such as glaucoma, ileus, or cardiac arrhythmias.

Antihistamine (H1-blocker). Similar to other antihistamines, it acts by blocking the H1 receptor and suppresses inflammatory reactions caused by histamine. The H1-blockers have been used to control pruritus and skin inflammation in dogs and cats, but efficacy for these indications has not been established. Other commonly used antihistamines include clemastine, chlorpheniramine, diphenhydramine, cetirizine, and hydroxyzine.

humans or animals receiving chlorpheniramine and may be of significance in veterinary patients:
ANTICOAGULANTS (heparin, warfarin): Antihistamines may partially counteract the anticoagulation effects of heparin or warfarin
MAO INHIBITORS (including amitraz, and possibly selegiline): May prolong and exacerbate anticholinergic effects
OTHER CNS DEPRESSANT DRUGS: Increased sedation can occur

Antihistamines have a high margin of safety. Sedation is the most common side effect, which is the result of inhibition of histamine N-methyltransferase. Sedation may also be attributed to blocking of other CNS receptors such as those for serotonin, acetylcholine, and alpha-receptors. Antimuscarinic effects (atropine-like effects) also are common, such as dry mouth and decreased GI secretions.