Smoking
Smoking is very harmful for the smoker as well as for his companions. It is the single biggest avoidable menace to good health. Smoking causes impotence and infertility besides narrowing and hardening of arteries. Smoking is also responsible for paralytic stroke, brain hemorrhage, angina, heart attack, chronic bronchitis, ulcers, cancer etc. Nicotine is a drug that occurs naturally in the tobacco leaf. In higher doses, nicotine, like other addictive drugs including heroin and cocaine is a poison. Nicotine has a powerful effect on the brain and the central nervous system. Nicotine causes an increase in the heart rate and in the rate of breathing - thereby making the heart work harder. Therefore, smoking kills more people every year than all other drugs and alcohol combined. Even non-smokers who are exposed to smoke suffer more than the smokers themselves, because of passive smoking.
Social attitudes towards smoking have changed dramatically in the civilized countries for the past few years and all smokers are aware of these. Smokers are becoming a lonely minority and they know their habit or addiction is harming those around them. Nearly all of them would like to quit smoking but one has to have a strong will to quit in one stroke and not slowly and steadily. Throw away all cigarettes, lighters, matches and ash-trays. Drink lots of water, fruit juices and herbal teas. Keep yourself busy. When the urge to smoke hits, take deep breath and hold for a few seconds. In fact, regular pranayama helps smokers quit and stay off cigarettes. Play with something like pencil or pain in hand. Eat several small meals. After quitting, you feel better and look better day by day in all respects by getting rid of diseases.

Tobacco /Zarda Chewing
This is certainly worse than alcohol and smoking as it is solely responsible for oral cancer, abdomen ulcers besides leading to many other diseases of the abdomen, lungs, liver, throat and brain. It makes the brain tissues sluggish, dull, numb and inactive making the person incompetent for brain work. Zarda / tobacco contains nicotine just like cigarettes. Nicotine is a harmful drug that takes you up then drops you flat. It makes you want more. It gets you hooked. Then you are in real trouble. Quit chewing , as suggested above, in one stroke and as soon as possible.

Pan Masala
Its effect is generally like zarda / tobacco and highly addictive in Nature. It is unfortunate that the habit of taking pan masala is acquiring a status symbol. If one knows the reality ( toxic ingredients and harmful effects) one will never like to touch it. Shun it as suggested above.

Alcohol
Apart from causing numerous social, moral and legal harms, ALCOHOL gives rise to various health problems including damaging the liver which is a very touch organ. It directly attacks and causes cirrhosis of the liver or cancer of the liver , gastritis,ulcer, nervous disorders etc.
The following anecdote vividly illustrates the evils of alcohol :

Drink is a demon that ruins man. Once, so the legend goes, the devil approached a man and said,"You are about to die. I can save you from death in any one of these 3 ways :
Kill your servant. Beat your wife. Drink this wine."
Let me think" said the man. "To kill my faithful servant is impossible ; to ill-treat my wife is ridiculous, I will drink the wine."
He drank the wine and being drunk he beat his wife and killed his servant who attempted to defend her.
When the servant died, wisdom dawned. But it was too late. The drunken man repented for life time.

Shed The Habit of Drinking Before Forced To Shed Tears.
"For he shall be great in the sight of the Lord ;and shall drink neither wine nor strong drink ; and he shall be filled with the Holy Ghost, even from his mother’s womb."

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