Haram To The Deen
Smoking spoils a person's acts of worship and reduces their rewards. For instance, it spoils the prayer, which is the pillar of Deen. Allah's Messenger
said: Whoever eats garlic or onion, let him avoid us and our masjid, and stay in his home. The angels are surely hurt by things that hurt the human beings(1)
Those with clean and undefiled fitrah (nature) have no doubt that the smell emanating from the mouth of a smoker is worse and more foul than that from the mouth of one who ate garlic or onion. Thus, a smoker is in between two options, either to harm the praying people and the angels with his foul smell, or miss the prayer in jama'ah.
Smoking also spoils fasting. Fasting is very hard for the smoker. As soon as the day is over, he hastens to break his fast on an evil cigarette instead of sweet dates or pure water. Even if he fasts through the month Ramadan, a smoker is reluctant to fast on other days. Thus he loses the great reward of those who fast even one day in Allah's way.
Harm To The Human Body
No one can deny the harm of smoking to the human body. The medical evidence for this is well established and overwhelming. Because of this, the law in the United States and many other countries requires including a warning on any smoking advertisement.
Smoking contains poisonous materials, such as nicotine, tar, carbon monoxide, arsenic, benzopyrene, etc., that the smoker swallows in small proportions. Their harm accumulates with time to result in a gradual killing of the human organs and tissues.
The hazards of smoking to the health are hard to enumerate. Cancer, tuberculosis, heart attacks, asthma, coughing, premature birth, infertility, infections in the digestive system, high blood pressure, nervousness, mouth and teeth diseases, etc., are among the many health hazards that have been strongly linked to smoking.
These diseases may not appear all at once, however a smoker is most likely to suffer from some of them, and his suffering increases as he grows older. Furthermore, statistics have established that smokers' age is, on the average, ten years less than other people's.
This is aufficient to prohibit smoking. Islam prohibits any action that causes harm to oneself or to other people. Allah (T) says (what means):
<Do not kill yourselves, Allah is indeed merciful to you.>(2)
<Do not cast yourselves, with your own hands, into destruction>(3)
And the Messenger
says: No harm may be inflicted on oneself or others.(4)
The feet of a human being will not depart, on the day of Judgement, from his standing before his Lord, until he is questioned about five things: his lifetime - how did he pass it, his youth - how did he used it, his wealth - where did he earn it and how did he spend it, and how did he follow what he knew.(5)
Whoever consumes poison, killing himself with it, then he will he consuming his poison in the hellfire, and he will abide in it permanently and eternally.(6)