Smoking is one of the leading causes of preventable deaths throughout the world. When smoke enters the human body it affects nearly each and every organ leading to many life-threatening diseases. The poisons from cigarette, which are basically tar and other chemicals when entering the blood stream, are responsible for narrowing the arteries as well as increase the blood clot formation. Both of these effects could lead to heart attack or stroke. 

Lungs are the most affected among all organs. The initial symptoms include colds, coughs, wheezing and asthma. However, these can progress to more fatal diseases such as pneumonia, emphysema and lung cancer. Smoking weakens stomach muscle which controls the lower of gullet. This makes the formation of acid in stomach to travel in wrong direction producing ulcers. 

Smoking also makes bones to become weak and brittle commonly called osteoporosis. Smoking also leads to various types of cancers, the most common being typey being leukaemia, lung, mouth, esophagus, larynx, throat, cervix, pancreas, colon, stomach, rectum and others as well. Furthermore, the immune system is weakened due to the high levels of tar and other chemicals in smoke. This makes the body more susceptible to more autoimmune diseases such as rheumatoid arthritis and multiple sclerosis.  

In addition, smoking causes erectile dysfunction in men due to the damage it causes to the blood vessels that supply blood to penis. Similarly, in women smoking lowers the estrogens levels which cause dry skin, thinning hair and memory problems.