Answered by Shaykh Faraz Rabbani
If, during Ramadan, I unitentionally swallowed a portion of a finger nail, what is the ruling on the validity of my fast?
Answer:
Walaikum assalam wa rahmatullah,
The fast is invalidated by any solid or liquid entering the body cavity through a considered point of entry (such as the mouth). It is not a condition that this be deliberate. [Mufti Rafi Usmani, Dabit al-Mufattirat; Ibn Abidin, Radd al-Muhtar]
Thus, swallowing a fingernail, even accidentally, would invalidate the fast and require making it up (without requiring a kaffara–expiation).
And Allah alone gives success.
Faraz Rabbani