The sublingual area – according to doctors – is the area of the body which most rapidly absorbs medicine. Hence the quickest treatment for some heart problems (angina) is a pill which is placed under the tongue, where it is absorbed directly and quickly, and the bloodstream carries it to the heart where it stops the angina attack.
This kind of medication does not break the fast because it is absorbed in the mouth and none of it enters the stomach. The one who uses it must be careful and make sure that he does not swallow any of it after it dissolves in the mouth and before it is absorbed.
It says in a statement of the Islamic Fiqh Council:
The following things are not regarded as breaking the fast: medicines that are placed under the tongue to treat angina pectoris and other problems, so long as one avoids swallowing anything that reaches the throat. End quote.
See: Majallat Majma’ al-Fiqh al-Islami (10/2/96, 454); Muftiraat al-Siyaam al-Mu’aasirah, by Dr. Ahmad al-Khaleel, p. 38, 39.