Cigarettes are one of few products which can be sold legally which can harm and even kill you over time if used as intended.

Currently there are ongoing lawsuits in the USA which aim to hold tobacco companies responsible for the effects of smoking on the health of long term smokers.

Benzene (petrol additive)
A colourless cyclic hydrocarbon obtained from coal and petroleum, used as a solvent in fuel and in chemical manufacture - and contained in cigarette smoke. It is a known carcinogen and is associated with leukaemia.

Formaldehyde (embalming fluid)
A colourless liquid, highly poisonous, used to preserve dead bodies - also found in cigarette smoke. Known to cause cancer, respiratory, skin and gastrointestinal problems.

Ammonia (toilet cleaner)
Used as a flavouring, frees nicotine from tobacco turning it into a gas, found in dry cleaning fluids.

Acetone (nail polish remover)
Fragrant volatile liquid ketone, used as a solvent, for example, nail polish remover - found in cigarette smoke.

Tar
Particulate matter drawn into lungs when you inhale on a lighted cigarette. Once inhaled, smoke condenses and about 70 per cent of the tar in the smoke is deposited in the smoker's lungs.

Nicotine (insecticide/addictive drug)
One of the most addictive substances known to man, a powerful and fast-acting medical and non-medical poison. This is the chemical which causes addiction.

Carbon Monoxide (CO) (car exhaust fumes)
An odourless, tasteless and poisonous gas, rapidly fatal in large amounts - it's the same gas that comes out of car exhausts and is the main gas in cigarette smoke, formed when the cigarette is lit. Others you may recognize are :

Arsenic (rat poison), Hydrogen Cyanide (gas chamber poison)

source: Health Education Authority (UK) - Lifesaver

SOURCE;quit-smoking-stop.com/harmful-chemicals-in-cigarettes