Abis bin Rabi'ah narrates from Sayyiduna Ali (may Allah be pleased with him) that every divorce is valid except the divorce of the mentally deranged (ma'tuh).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الخلع والطلاق / 15111
Imam Malik (may Allah have mercy on him) said that Sa'id bin al-Musayyib and Sulayman bin Yasar (may Allah have mercy on them both) were both asked about the divorce of one who is intoxicated. They said: "When an intoxicated person gives a divorce, his divorce is valid; and if he kills, he is killed." Imam Malik (may Allah have mercy on him) said: "This is our ruling."
Ibrahim (may Allah have mercy on him) said that the divorce and the freeing of a slave by an intoxicated person are valid.
Al-Hasan al-Basri (may Allah have mercy on him) said that the divorce and the freeing of a slave by an intoxicated person are valid, but his buying and selling are not valid.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الخلع والطلاق / 15112