It is prescribed for sick people not to fast in Ramadaan, if fasting will cause harm or make the sickness worse, or if they need treatment during the day in the form of medicine or pills that must be swallowed, because Allaah says (interpretation of the meaning): 

“and whoever is ill or on a journey, the same number [of days which one did not observe Sawm (fasts) must be made up] from other days”[al-Baqarah 2:185] 

And the Prophet (peace and blessings of Allaah be upon him) said: “Allaah loves people to avail themselves of His concessions (rukhsah) just as He hates them to commit sin.” According to another version, “As He loves His commands to be obeyed.” 

With regard to taking blood from veins for testing etc., the correct view is that this does not break the fast, but if it is done often, it is better to leave it until night-time. If it is done during the day then to be on the safe side that day should be made up, because this is akin to cupping.”

(Fatwa of Shaykh Ibn Baaz (may Allaah have mercy on him), from Fataawa Islamiyyah, vol. 2, p. 139) 

Sickness is of various kinds: 

1 – That which does not affect the fast, such as a light cold or mild headache or toothache, and the like. In this case it is not permissible to break the fast, even though some of the scholars permitted that because of the aayah (interpretation of the meaning): 

“and whoever is ill…”[al-Baqarah 2:185] 

But we say that this ruling is based on a reason, which is that not fasting is easier. If that is the case, then we would say that not fasting is better, but if fasting does not have an effect on him then it is not permissible to break the fast, and he has to fast. 

2 – If fasting is difficult for the sick person, but it does not harm him, then it is makrooh for him to fast and it is Sunnah for him to break his fast. 

3 – If fasting is difficult for him and will cause him harm, such as a man who has kidney disease or diabetes and similar cases where fasting will cause harm. In this case fasting is haraam. Hence we may see that some mujtahids and sick people make a mistake when fasting is difficult for them and may harm them, but they refuse to break their fast. We say that they are mistaken because they refuse to accept the kindness of Allaah and the concession that He has given to them, and they are harming themselves, although Allaah says (interpretation of the meaning): 

“And do not kill yourselves”[al-Nisa’ 4:29] 

al-Sharh al-Mumti’ by Shaykh Ibn ‘Uthaymeen, vol. 6, 352-254.