Can you drink alcohol on Zyrtec?
An occasional drink on Zyrtec is not a chemical emergency, but MedlinePlus advises avoiding alcohol because it can add to the drowsiness cetirizine already causes.
Why
Zyrtec's active ingredient is cetirizine, an antihistamine. MedlinePlus notes it "may make you drowsy" and explicitly says to "avoid drinking alcoholic beverages while taking this medication" because "alcohol can add to the drowsiness caused by this medication." Both are central nervous system depressants, so the sedative effects stack — the practical risk is being more impaired, sleepy, or dizzy than you would expect from either one alone, which matters for driving or operating machinery.
What to watch for
Notice how cetirizine affects you before adding alcohol — some people get drowsy on it, some do not. If you do feel sleepy or dizzy, do not drive or operate machinery, and skip the alcohol. A single drink well-spaced from your dose is lower risk than drinking heavily or close to when the medicine peaks. Ask a pharmacist or doctor first if you take other sedating medicines (sleep aids, opioids, benzodiazepines, muscle relaxants), have liver problems, are pregnant or breastfeeding, or are older, since sedation adds up faster.
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.