As mentioned earlier, patients are exempt from fasting. But some, for whatever reasons, do decide to observe fasting. For physicians treating Muslim patients, the following guidelines are suggested.
a. Diabetic Patients: Diabetics who are controlled by diet alone can fast and hopefully with weight reduction, their diabetes may even be cured or at least improved. Diabetics who are taking oral hypoglycemia agents like Daonil along with the diet should exercise extreme caution if they decide to fast. They should reduce their dose to one-third, and take the drug not in the morning, but with Iftar in the evening. If they develop low blood sugar symptoms in the day time, they should break the fast immediately. Diabetics taking insulin should not fast. If they do, at their own risk, they should do so under close supervision and make drastic changes in the insulin dose. For example, eliminate regular insulin altogether and take only NPH in divided doses after Iftar or before Sahar. Diabetics, if they fast, should still take a diabetic diet during Iftar, Sahar and dinner. The sweet snacks common in Ramadan are not good for their disease. they should check their blood sugar before breakfast and after ending their fast.
b. Hypertensive or Cardiac Patients: Those who have mild to moderate high blood pressure along with being overweight should be encouraged to fast, since fasting may help to lower their blood pressure. They should see their physician to adjust medicines. for example, the dose of water pill (diuretic) should be reduced for fear of dehydration and long acting agents like Inderal LA or Tenormin can be given once a day before Sahar. Those with severe hypertension or heart diseases should not fast at all.
c. Those with Migraine Headache: Even in tension headache, dehydration, or low blood sugar will aggravate the symptoms, but in migraine during fasting, there is an increase in blood free fatty acids, which will directly affect the severity or precipitation of migraine through release of catecholamines. Patients with migraines are advised not to fast.
d. Pregnant Women (Normal Pregnancy): This is not an easy situation. Pregnancy is not a medical illness, therefore, the same exemption does not apply. There is no mention of such exemption in Quran.However, the Prophet said the pregnant and nursing women do not fast. This is in line with God not wanting anyone, even a small fetus, to suffer. There is no way of knowing, the damage to the unborn child until the delivery, and that might be too late. In my humble opinion, during the first and third trimester (three months) women should not fast. If however, Ramadan happens to come during the second trimester (4th-6th months) of pregnancy, a women may elect to fast provided that (1) her own health is good, and (2) it is done with the permission of her obstetrician and under close supervision. The possible damage to the fetus may not be from malnutrition provided the Iftar and Sahar are adequate, but from dehydration, from prolonged (10-14 hours) abstinence from water.
Therefore it is recommended that Muslim patients if they do fast,do so under medical supervision.
source: Shahid Atar, MD. Islamic Medicine:Medical Aspects of Islamic Fasting