Smoking cigarettes is very hard to stop. For many people smoking is a part of their daily lives.
Cigarettes contain nicotine and nicotine is a very addictive drug. It only takes seven seconds for the nicotine to reach the brain after it is inhaled. The brain and body quickly get used to the effects of nicotine and need more cigarettes more often to avoid withdrawal symptoms like tiredness, irritability, hunger etc. If you know someone who experiences these symptoms when they do not smoke for a period of time, it means that they are addicted to tobacco. The effect that are felt from smoking only lasts a few minutes, but the impact on health can be forever.
Nicotine itself is a toxic substance and cigarettes contain 4000 chemicals, at least 40 of which are cancer causing. How smoking affects the body depends on a number of factors including the length of time the person has been smoking, how many cigarettes are smoked in a day, age, and any medical conditions.
All you can do is offer support and try not to pressure them into doing something they do not want to do. Later down the road, your friend or loved one may think about it for a number of reasons. Quitting smoking is an individual experience and by them simply thinking about wanting to quit means that they are on their way to making that change to become smoke-free.