One response from the fatwa databank is as follows:
A general rule of the Islamic Shari’ah is that it is haram for the Muslim to eat or drink anything, which may cause his death, either quickly or gradually, such as poisons, or substances, which are injurious to health or harmful to his body. It is also haram to eat or drink large quantities of a substance if large quantities of it cause illness. For the Muslim is not entirely his own master; he is also an asset to his religion and his ummah (the Muslim nation), and his life, health, wealth, and all that Allah has bestowed upon him are a trust with him, which he is not permitted to diminish. Says Allah Subhanahu wa Ta’ala: And do not kill yourselves; indeed, Allah is ever Merciful to you. (4:29)
He also says: And do not be cast into ruin by your own hands…. (2:195)
And His Messenger (peace be on him) said: “Do not harm yourself or others.” (Reported by Ahmad and Ibn Majah.)
As an application of this principle, we may say that if it is proved that the use of tobacco is injurious to health, it is haram, especially for a person whose physician has advised him to stop smoking. Even if it is not injurious to health, it is still a waste of money, spent neither for religious nor for secular benefit, and the Prophet (peace be on him) forbade wasting of property. (Reported by al-Bukhari.) This becomes the more serious when the money is otherwise needed for the sustenance of oneself or one’s family.
Another response from the fatwa databank:
Sheikh Yusuf Al-Qardawi, in his book, “Halal and Haram in Islam,” says it is haram. Last night, I heard a news piece on the BBC saying that the latest research is that those who smoke in their later years will suffer memory loss. This is besides the usual statistics that are very well-known about the cancer-inducing effects of nicotine, etc, such as those proven in rabbits.
And Allah (swt) knows best.