Praise be to Allah.
Death is one of the things that every person goes through, when his soul is taken away and he leaves behind the restrictions of the physical body and its connection with earthly matters, and passes into the vast realm of the Hereafter. That is the moment when the soul departs from the body, after which it has no control over it.
In at-Tabaqaat al-Kubra by Ibn Sa‘d (4/260) and al-Mustadrak by al-Haakim (5915) it is narrated that ‘Awaanah ibn al-Hakam said: ‘Amr ibn al-‘Aas used to say: “How amazing it is that the one whom death approaches when he is still in his right mind cannot describe it”. Then when his own death approached, his son ‘Abdullah ibn ‘Amr said to him: “O my father, you used to say, ‘How amazing it is that the one whom death approaches when he is still in his right mind cannot describe it’; describe death to us whilst you are still in your right mind.” He said: “O my son, death is too great to be described, but I shall describe something of it to you: I feel as though the mountains of Radwa are on my neck, and I feel as though there are sharp thorns inside me, and I feel as though my soul is emerging through the eye of a needle. End quote.
As this transition is so difficult, all remedies and painkillers are a waste of time. Death is a process of the unseen in which the soul is separated from the body, and anesthetics and painkillers only serve to reduce physical pain that has to do with the tangible and physical world. The realm of death and the hereafter lies beyond such means and is not subject to regular treatment and remedies.
Based on that, what appears to us to be the case here is that it is not allowed to use anaesthesia at the time of death, for the following reasons:
1. The basic principle concerning anaesthesia is that it is not allowed except in cases of necessity, and there is no need for it in this case. We have stated above that anaesthesia does not remove the pain of death; rather it has no nothing to do with the process of death that belongs to the world of the unseen. As that is the case, using anaesthesia falls under the heading of doing something that is not allowed, without any legitimate shar‘i reason for permitting it; we do not know that doing that will bring any benefit or that there is any need for it, nor do we think it most likely that that will be the case this situation.
See also the answer to question no. 46050
2. It is not possible for anyone to know the exact time of death. Anaesthesia may cause physical harm or lead to complications. In fact some doctors say that it is a type of poison. Hence it is unacceptable to introduce something that is definitely harmful to try to ward off something that is mere conjecture; in fact that is dealing with something the true nature of which we do not know and we have no experience of it at all, and we do not know whether anaesthesia is beneficial in this case.
Moreover, in the case of the righteous slave whose life Allah is bringing to an end with obedience and worship, whom we see turning to Allah, may He be exalted, and remembering Him, we expect him to utter the Shahaadah (testimony of faith) that will be stored up for him with his Lord, and anaesthesia may deprive him of this virtue. It was narrated that Mu‘aadh ibn Jabal (may Allah be pleased with him) said: The Messenger of Allah (blessings and peace of Allah be upon him) said: “The one whose last words are Laa ilaaha ill-Allaah will enter Paradise.”
Narrated by Abu Dawood, 3116.
Ibn Hajar al-Haytami (may Allah have mercy on him) said: That is, without any punishment at all, or the punishment he deserves will be reduced in some ways. We interpreted it in this manner because every Muslim will inevitably enter Paradise, even if he is punished, so the report would not mean anything if having as one’s final words the testimony of faith guarantees admittance to Paradise, unless there is a particular advantage in doing so, and this advantage is that either he will enter it with those who are saved, without any punishment, or Allah, may He be glorified, will reduce some of the punishment that he deserves, and admit him to Paradise before the time when he would have deserved that if he had not ended his life with these words.
End quote from az-Zawaajir, 2/333
And Allah knows best.