Nicotine is what makes cigarettes so addicting. Nicotine acts on acetylcholine receptors in the brain, which help to control attention and memory. Nicotine also boosts levels of the neurotransmitter dopamine and the hormone adrenaline. Eventually the brain becomes accustomed to these changes, so when people attempt to stop smoking, they can experience symptoms of withdrawal such as cravings, headaches, dizziness, anxiety, and irritability.