It is not a drug with any significant risk of acute toxicity even in much higher doses than the therapeutic dose, so I am not sure why you feel that it should not be OTC?

https://toxnet.nlm.nih.gov/cgi-bin/sis/search/a?dbs+hsdb:@term+@DOCNO+7739 Gives more info about reported cases of overdoses. A drug such as paracetamol/acetaminophen is much more commonly the cause of death and is OTC.

But isn't exceeding the minimum toxic dose would result in toxicity? We know that the maximum allowed dose (minimum toxic dose) is 10 mg / day, so small overdose would result in some form of toxicity, isn't that true? Otherwise, why they would write that the maximum dose is 10 mg /day if no form of toxicity would appear exceeding this dose?

By the way I'm speaking in terms of therapeutic window, not therapeutic index.

Dear Mostafa,

First, therapeutic Index is just a parameter to express therapeutic window. Both terms talk the same thing as to me. They tell you the safety of a particular drug.

Cetirizine is a potent antihistamine. But, that doesn't mean it has a low Safety index. It rather has fewer side effects than the first generation antihistamines.

Dr.Gizat, I have attached an image to illustrate my point, different terms.

[Comparative study between 10 and 20 mg of cetirizine in the symptomatic treatment of seasonal allergic rhinoconjunctivitis].

Article [Comparative study between 10 and 20 mg of cetirizine in the...

These links may help you

Therapeutic index of H1-antihistamines: example of cetirizine.

Article Therapeutic index of H1-antihistamines: Example of cetirizine

http://onlinelibrary.wiley.com/doi/10.1034/j.1398-9995.2000.00010.x/pdf

General discussion ± Round-table meeting ``The therapeutic index of antihistamines'' Amsterdam, 5±6 May 1998

https://pdfs.semanticscholar.org/9eff/210276183566a4ce53db1efc6e16ff4423ad.pdf

Comparative analysis of the cardiotoxicity proclivities of second generation antihistamines in an experimental model predictive of adverse clinical ECG effects.

https://www.ncbi.nlm.nih.gov/pubmed/8720304

Evaluation of the potential cardiotoxicity of the antihistamines terfenadine, astemizole, loratadine, and cetirizine in atopic children

Article Evaluation of the Potential Cardiotoxicity of the Antihistam...

But Dr.Mohamed, this is about the therapeutic index, not the therapeutic window which is indicated by 10 mg as the maximum dose. By therapeutic window, I mean the minimum effective dose, and Maximum tolerated dose (Minimum Toxic dose) which you'll find is 10 mg / day. So, I was wondering why this is the upper limit? Is this because that above this limit, no more benefit will be added regardless of the toxic profile?

The therapeutic index of cetirizine is not indicated by the recommended maximum dose of 10 mg/day. I belive the recommended dose was chosen to be 10 mg/day since it had the same efficacy as 20 mg/day in several randomized controlled trials. Not due to any toxicity.

Sorry, I was referring to the therapeutic window, not the therapeutic index

Cetirizine uses

Upper respiratory allergies: Temporarily relieves symptoms of upper respiratory allergies.

Urticaria: Relieves itching due to urticaria.

Seldom a patient will go to a doctor for this symptoms, and if patient tries to reach the doctor for this symptoms will be by calling, so thats why it is OTC drug, no need for prescription, and therapeutic index although narrow nonetheless symptoms are not that extreme, with respect to pharmacists, they should advise the patient to take one tablet per day, and counsel everypatient that purchases this drug on side effects and on not to use higher doses.

Cetrizine is one of the most widely studied second generation antihistamine and its safety profile has been well studied..Newer antihistaminics are compared with it...........May be this is a reason