Siman 532

The Rambam in the seventh chapter ruled that it is permitted to trim fingernails during Chol HaMoed, even with a tool, in the manner they are trimmed on a weekday. The Semag wrote similarly. This is also the opinion of the Rif and the Rosh, who explained that that which we say in the Gemara (17b) that with a gnosteri it is forbidden—this refers to mourning and not to Chol HaMoed. However, the Tur brought opinions that there are those who only permitted with a knife, and not with scissors. And there is one who forbids even with a knife, but only with his hands.

Therefore, the custom among us is to be stringent regarding the cutting of nails during chol hamoed, whether with a knife or with scissors, unless it is for the purpose of a mitzvah, such as a woman for her immersion. And certainly those who were explained to be permitted to shave during chol hamoed. Similarly, one who cut his nails before the festival, and had nothing to cut on the eve of the festival, and they grew during chol hamoed—he is permitted to cut them in any manner he desires (Magen Avraham). And it appears to my humble opinion that in a case of pressing need, one who was very preoccupied before the festival—he may cut his nails during chol hamoed, since such is the opinion of four pillars of the world. And it is permitted to take all types of medicines during chol hamoed. And even though through this he will not be able to delight in eating and drinking, nevertheless his healing takes precedence, even if it is not a complete illness. (So it appears to me, for it is not because of the prohibition that the Magen Avraham in subsection 2 wondered from siman 496, see there. This is the wording of the Tosefta brought by the Rif and the Rosh in the first chapter of Moed Katan: One may drink dung water..., see there. Examine and you will find it simple.)