Alhamdulillah ALLAH SWT has made it easy for His slaves and has only enjoined fasting upon those who are able, and has excused those who are unable to fast for a sound reason. The reasons for which one may be excused from fasting are as follows:
The ill and sick
Ibn Qudaamah said: the scholars are agreed that it is permissible for the sick person not to fast in general. The basis of that is the ayah in which ALLAH says “but if any of you is ill or on a journey, the same number (should be made up) from other days”[Al-Baqarah 2:184]
If a sick person fears fasting may increase the severity of sickness or slow down recovery is exempted from fasting. It is Sunnah for one to abstain from fasting until he is alright. ALLAH SWT has allowed a sick person to refrain from fasting as a mercy from Him, and a convenience to the sick. But if a healthy person fears difficulty or tiredness, it is not permissible for him to break his fast, if all that happens to him when he fasts is that he becomes tired.
The basis for this exemption also depends on the above mentioned verse of the holy Quran. Anas (may ALLAH be pleased with him) said: “I travelled along with Prophet PBUH during Ramadan, and the fasting person would not criticize the one not fasting, nor the one not fasting criticize the fasting person.”[Sahih Bukhari]
If fasting on a journey produces hardship for the believer then it is not righteous to fast rather it is better and more beloved to ALLAH SWT that he breaks the fast. Prophet PBUH said “It is not righteousness that you fast on a journey.” [Sahih Bukhari]
The ease which has been granted to the traveller is applicable till the Day of Judgment irrespective of how comfortable the journey is, nowadays. The person who criticizes the one who leaves the fast while on journey is in fact criticizing the allowance given by ALLAH SWT himself.
Pregnancy and breastfeeding women
Anas Ibn Maalik al-Ka’bi (may ALLAH be pleased with him), said that the Messenger of ALLAH (Peace and Blessings of ALLAH be upon him) said: “ALLAH has relieved the traveller of fasting and half of prayer, and He has relieved the pregnant and breastfeeding woman of fasting.”
If such a woman fear for her or her child’s health and life, then she can leave the fast and for every missed fast, she should feed one poor person and she is not required to compensate for the missed fasts by fasting later on. Once Ibn Umar’s wife asked him about this, so he replied “You leave the fast, and for every day of missed fast, feed one poor person and there is no recompense due on you.” [Daarulqutnee]
Senility and old age
Senility and old age refers to one who is old and has lost his strength, or who is approaching death, so that every day he becomes weaker, until he dies, or who is suffering from a terminal or incurable illness and has no hope of recovery. The evidence that it is prescribed for such people not to fast is the following ayah (interpretation of the meaning):
“And as for those who can fast with difficulty, (e.g. an old man), they have (a choice either to fast or) to feed a miskeen (poor person) for every day”[Al-Baqarah 2:184] and said “It is the old man who is unable to fast, so he refrains from fasting and instead feeds a poor person with half a Saa (a measure which is equal to 2 Kilo and 40grams) of wheat.”[Sahih Bukhari]
Anas Ibn Malik RA became too weak one particular year, so he prepared a large dish of Thareed (a broth of crumbled meat and bread) and invited thirty poor people who came and ate to their fill.
Menstruating and post natal bleeding women
Menstruating women and women having post-natal (after delivery) bleeding are not allowed to fast and it is obligatory to compensate by making up for the days later on [Muslim]
The above mentioned allowances are given by ALLAH SWT to his slaves in certain circumstances and hence it is advised to utilise appropriately. Prophet PBUH said “indeed ALLAH SWT loves to give allowances, as HE loves to give HIS obligations” [Ibn Hibban]