Can zyrtek cause behavioral changes, and rapid heart beat in children? evaluating whether to give this to my son for dust mite allergy

Possible: but not common. Sleepiness and dry mouth are the most commonly reported side effects. Irritability, behavioral problems and rapid heart rate have occurred in less than 1% of pediatric patients in a large study.

See a good allergist: Zyrtec (cetirizine) is the most potent of the new antihistamines and while it is very effective as an allergy medication, it does have side-effects, although rarely seen. For instance, 10% of kids/adults have a very hard time waking up in morning if on this med, or feeling drained, tired. My family has experienced this first hand....so behavioral changes is certainly possible I expect. Talk to your allergist.