The only conceivable "harmful" aspect is nicotine...
There's the damning ingredient. Nicotine. Remember, smoking is a DFing offense for it's links to spiritism as much as it is for it being addictive or nasty habit. The drug that links the smokers to the demons is nicotine, so whether you roll your own, buy cigarettes from RJ Reynolds or Phillip Morris, your cigars from Cuba, or get your nicotine hit through an e-cig, it's still a DFing offense.
Per the WTS, that is:
Watchtower June 1, 1973
13 "The drug . . . that causes the addiction is nicotine. . . . Within a minute or two after a person ‘takes a drag’ on a cigarette, nicotine is present in the brain. But 20 to 30 minutes after the ‘last drag,’ most of the nicotine has left the brain for other organs . . . . This is just about the time when the smoker needs another cigarette. . . . When there is no nicotine, the body ‘hungers’ for it. So much so that the body sometimes becomes ‘sick’ without it. Withdrawalsymptoms—a sick feeling—begin. . . . Some of these symptoms are drowsiness, headaches, stomach upsets, sweating, and irregular heart beats."
14 Even worldly governments have been moved to issue serious warnings against the danger of tobacco use. Do, then, persons who have not broken their addiction to tobacco qualify for baptism?
15 The Scriptural evidence points to the conclusion that they do not. As has been explained in other issues of this magazine, the Greek word phar·ma·ki´a used by Bible writers and translated "practice of spiritism" or "spiritistic practices" has the initial meaning of "druggery." (Gal. 5:20; Rev. 9:21) The term came to refer to spiritistic practices because of the close connection between the use of drugs and spiritism. Tobacco was also used initially by the American Indians in this way. It can properly be placed, therefore, in the category of addictive drugs like those that provided the source for the Greek term phar·ma·ki´a.
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31 "‘And I will come near to you people for the judgment, and I will become a speedy witness against the sorcerers [phar·ma·kous´—according to the Greek Septuagint], and against the adulterers, and against those swearing falsely, and against those acting fraudulently with the wages of a wage worker, with the widow and with the fatherless boy, and those turning away the alien resident, while they have not feared me,’ Jehovah of armies has said."
32 Note that the first ones whom Jehovah mentions as the targets of his speedy witness regarding their wrongdoing are the "sorcerers." The Greek SeptuagintVersion, as translated by Alexandrian Jews before Christ, rendered "sorcerers" by the Greek word phar·ma·kous´. This is the same word used in Revelation 21:8, where some translators render it as "sorcerers," but the NewWorldTranslation renders it as "those practicing spiritism." The ancient sorcerers indeed practiced spiritism. The Greek word applied to them literally means "druggers," not "druggists" such as "pharmacists." The ancient sorcerers were the drugpushers of their day.
33 The pre-Christian Greek SeptuagintVersion uses the related Greek word phar´ma·kon (meaning literally "drug," but translated as "sorcery") at least five times. Idolatrous Queen Jezebel of ancient Israel practiced such phar´ma·kon (in the plural number) or "sorcery." (2 Ki. 9:22, LXX) She was executed by King Jehu acting as Jehovah’s executioner. Those who patronized the professional "sorcerers" or practicers of spiritism also participated in spiritistic practices and were condemned.
34 It is little wonder, then, that, in these days of widespread addiction to drugs and the growing use of tobacco, those indulging in such things should come under judicial observation. Jehovah God, the Supreme Judge, is at his spiritual temple and is specially scrutinizing those who profess to worship him in that holy place. He has promised to be a speedy witness against the sorcerers or the practicers of spiritism, which from ancient times onward had a connection with habit-forming, enslaving drugs.
35 Do we want to have Jehovah God be a speedy witness against us as addicts to drugs or other habit-forming injurious weeds, things that expose us to the influence of the spirit demons? Jehovah’s judgment against such addicts during the oncoming "great tribulation" will mean their destruction. (Rev. 21:8) Most assuredly, Jehovah God does not want such addicts among the congregation of his Christian witnesses now in this "conclusion of the system of things." Of the promised "New Jerusalem," Revelation 22:15 says: "Outside are the dogs and those who practice spiritism [the druggers, KingdomInterlinear translation] and the fornicators and the murderers and the idolaters and everyone liking and carrying on a lie."
36 Therefore, with a feeling of a deep sense of responsibility to Jehovah God, these instructions are issued. It is accordingly the duty of the elders, as spiritual overseers of God’s flock, to see to it that such undesirable elements are not accepted as approved, baptized members of congregations of Jehovah’s Christian witnesses.