Answers by TheMediTary.Com - Last updated: 12-Jul-2023
If your allergy is particularly bad then yes you can take them on the same day, as there are no known interactions. This is a therapeutic duplication and it is usually recommended to take only one antihistamine at any time, however if you have been prescribed to take both together then it is appropriate.
If you do experience any odd symptoms please see your doctor.
You can read more about Allegra and Zyrtec here:
https://www.drugs.com/medical-answers/allegra-zyrtec-difference-3130012/