Is it permissible for a sick person to use one nostril for sniffing water during ablution?

Istinshaq (sniffing water into the nose) is the act of drawing water into the nose with one's breath. Its most complete form is drawing it to the furthest part of the nose, and its sufficient form is drawing it to its inner part, even if only a little. The default is to use both nostrils, based on the hadith from Abu Hurairah: "When one of you performs ablution, he should sniff water into his nostrils, then expel it."

If one's nose is blocked, or if drawing water with both nostrils simultaneously causes harm, then drawing it with one nostril is sufficient; this is based on the Almighty's saying: "Allah does not burden a soul beyond its capacity." Sheikh Ibn Jibreen was asked about the case of one nostril being blocked during istinshaq, and he replied that the initial istinshaq is sufficient, and it is not necessary to repeat it after the blockage is removed.