Gateway drugs like cigarettes and alcohol can open the door to trying more dangerous substances. Cigarettes contain nicotine and can lead to health issues like cancer, heart attacks, and strokes. Alcohol is also dangerous if consumed in large amounts, as it can result in conditions like alcoholism, fatty liver, and cirrhosis over time. Both cigarettes and alcohol are addictive substances that should be avoided, especially for teenagers, as they can potentially lead to other drug use.