Generally you should not take Benadryl (diphenhydramine) and Zyrtec (cetirizine) together on your own — they're both antihistamines, and the NHS says not to take two antihistamines at once unless a doctor recommends it.
Both drugs are antihistamines that block the same histamine (H1) receptors, so combining them doubles up on the same drug class without adding much benefit. The bigger concern is additive drowsiness and anticholinergic effects: Benadryl is a sedating first-generation antihistamine, and although Zyrtec is "non-drowsy," the NHS notes it's still possible to feel sleepy after taking it, so stacking them can increase sedation, dry mouth and difficulty urinating. The NHS does describe one supervised exception — doctors sometimes recommend taking 2 different antihistamines together for a few days for a severe, itchy skin rash — but it explicitly says: "Do not take 2 antihistamines together unless recommended by your doctor."
Don't combine them yourself for everyday allergy relief; pick one. If a doctor or pharmacist has told you to use both, follow their exact schedule and the dosing on each label, and don't exceed the max dose of either. Watch for excessive drowsiness, dizziness, confusion, dry mouth, blurred vision, constipation, or trouble urinating — and don't drive or use machinery if affected. Avoid alcohol (MedlinePlus warns it adds to diphenhydramine's drowsiness) and other sedating medicines such as sleep aids and opioids/codeine, which add to the sedation. Be extra cautious checking labels: many cough/cold and "PM" products already contain diphenhydramine or another antihistamine, so you can double-dose by accident. Older adults are especially sensitive to the sedative and anticholinergic effects — MedlinePlus says diphenhydramine generally should not be used in older adults except for serious allergic reactions. Always check with a pharmacist or doctor before combining, particularly if you have glaucoma, an enlarged prostate, urinary problems, or a heart condition. Get urgent help for severe drowsiness, fast/irregular heartbeat, or signs of overdose.
This is general reference, not medical advice, and not a guarantee of safety. Interactions depend on your doses, health conditions, and other medicines. Always confirm with your pharmacist or doctor before combining products, and follow the dosing on each label.