Adults should not take more than 10 mg of cetirizine (usually one standard tablet) in any 24‑hour period unless a doctor specifically tells them to do so.

Quick Scoop

  • For most adults under 65: the usual and maximum dose is 10 mg once a day (often 1 tablet).
  • Taking more than 10 mg in 24 hours increases the risk of side effects (sleepiness, dizziness, fast heart rate, confusion) without giving extra benefit.
  • Older adults or people with kidney problems are often advised to stay at 5 mg per day and not go higher without medical advice.
  • For children, the safe maximum is lower and varies by age, so you must follow the package or a doctor’s instructions.

If you’ve already taken more than 10 mg in the last 24 hours or feel unusually drowsy, dizzy, agitated, or unwell, contact a doctor, poison center, or emergency service right away.

Typical daily limits by age (general guidance)

  • Adults and children 6 years and older:
    • Usual dose: 5–10 mg once daily.
    • Maximum : 10 mg in 24 hours.
  • Children 2–5 years:
    • 2.5–5 mg total in 24 hours (often split as 2.5 mg twice daily if needed).
  • Adults 65+ or with kidney issues:
    • Often 5 mg once daily, and they should not increase without speaking to a clinician.

Important safety notes

  • Do not double up because symptoms are bad; instead, talk to a doctor about adding or changing allergy medicines (for example, nasal sprays or other non‑sedating antihistamines).
  • Avoid alcohol and other sedating medicines (like some sleep aids, opioids, or anxiety meds), which can worsen drowsiness and slow breathing.
  • Different brands (like Zyrtec and some “Benadryl Allergy” products) don’t all contain cetirizine, so always check the active ingredient to avoid accidentally taking multiple antihistamines at once.

TL;DR:
For most adults, the safe ceiling is one 10 mg cetirizine tablet in 24 hours ; if you think you need more than that, or if you’ve already taken more, get professional medical advice urgently.

Information gathered from public forums or data available on the internet and portrayed here.