Introduction: Fasting is the Islamic worship of man in order to approach God. There is a direct relationship between fasting, abstaining from eating and drinking, and man’s health as well as ill-health. Therefore, through fasting the religious and medical issues are interwoven with each other; so that man can maintain his physical heath by performing a religious worship.Findings: Medical propositions which are based on experience can prove what would be harmful or urgent for body to take. In contrast, religious propositions relying on the meta-physical world are set for all people in every place at every time. Fasting is a religious term and in medicine, it is merely dealt with abstaining from eating and drinking. The concept of harmfulness means making a defect or disorder in the system of body, the recognition of which is the responsibility of medicine and the decision about which is within the responsibility of man. Conclusion: Medical science can determine the effects and consequences of thirst and hunger in the specific hours of the month of Ramadan. According to the religious perspective, it has been emphasized that fasting is for maintaining man’s health, and the conditions in which there is the probability of harmfulness for man’s health due to fasting, man shouldn’t fast. As a result, medical science can determine the interval of fasting.