Nicotine is a stimulant drug that affects your body and mind. When you ingest nicotine, your brain releases more of the neurotransmitter dopamine, which makes you feel more focused and happier. Nicotine also increases your heart rate, blood pressure and blood flow. If you obtain nicotine from cigarettes, you also take in carbon monoxide, which decreases the amount of oxygen that your blood is able to deliver to parts of your body such as your muscles.
Consuming nicotine vastly limits your body's ability to take in and distribute oxygen to your muscles. Cigarettes limit your oxygen intake by damaging your lungs as well as introducing carbon monoxide into your body which replaces oxygen in your blood. These lower amounts of oxygen make it more difficult for you to do your best when lifting weights.