Introduction: Despite the advice "Drink at least eight glasses of water a day" is ubiquitous and the nutrition resources insist on that, there are not enough scientific documents to support this advice, and according to the traditional Iranian medicine (TIM) it is even very harmful to drink such amount of water a day. TIM practitioners believe that, water drinking is just allowed when one is really thirsty. In TIM textbooks, Forbidden drinking times and the adverse effects of drinking water more than the body need are explained in detail. In this study, the viewpoints of conventional nutrition science and TIM about this issue are compared and discussed. 

Methods: Reviewing available references from TIM as well as the books, papers and case reports from the conventional medicine about water drinking, we made a comparative study in this field that is presented.

Result: Valtin H. et al in a paper published in 2002, challenged the recommendation of "Drink at least eight glasses of water a day". He believes that not only there is not enough evidence for this recommendation, but also this suggestion may be harmful. After publication of this article the Institute of Medicine revised its previous protocol and changed this advice and suggested the "thirst" is the best symptom to show our need to water; which confirms the rule that had been mentioned by Avicenna. Another important point to consider about nutritional guidelines is that these items are prescribed the same for all people in the conventional medicine; however the drinking water rules are different in according to people temperaments (mezaj) in the TIM.

Conclusion: In conclusion the authors recommend not following the hydrotherapy or drinking suggestions without scientific evidence-based reports and encourage researchers to investigate possible side effects of excessive water intake in animal models and then human studies.