Praise be to Allah the Lord of the Worlds.
It is lawful for a woman to lactate by using medications or else unless it entails harm, and this milk establishes the rulings of Sharia on fosterage (Al-Ridi`). For example, marital prohibition, privacy with a woman and the like.
If a woman-married or not-breast-feeds a baby (who is less than two years old) five separate breast-feedings, it becomes her baby by fosterage. This is whether her milk was made naturally or by medications.
The prohibition due to fosterage is established for the woman alone if she isn`t married or married, but her milk was made by medications. The suckled baby-girl becomes the foster-daughter of the woman`s husband after he has an intercourse with her (woman). This is attested to by the verse in which Allah the Almighty says, "and these foster-sisters include those suckled by a woman with whom the man has had intercourse …."{An-Nisaa`, 23}.
Marital prohibition, with all its rulings, is established between the husband of the woman and the girl suckled by her once he has an intercourse with that woman. This is on the basis that she is his wife`s foster-daughter and not his. Al-Mawardi says, " The girl suckled by a woman with whom the man has had an intercourse refers to wife`s daughter or her son`s daughter on grounds of consanguinity or fosterage."{Hashiyat Al-Khalyoubi}. It is stated in the book "Al-Bayaan Wat-tahseel": " A woman`s daughters from an earlier marriage as well as her foster-daughters are prohibited in marriage for her husband once he has an intercourse with her." However, the girl suckled by a man`s wife isn`t prohibited for his descendants (Furu`), ascendants (Usool) and brothers, because she isn`t his foster-daughter. And Allah the Almighty knows best.