Can I give benadryl to my 11 year old son who had a Zyrtec dose 12 hours before?

It's generally advisable to avoid giving Benadryl and Zyrtec (cetirizine) close together, as both are antihistamines and can increase the risk of side effects like drowsiness. Given that your son took Zyrtec 12 hours prior, it may still be in his system. Always consult with a healthcare professional or pediatrician before combining medications to ensure it is safe for your child's specific situation.