Some of the things listed above: drugs, alcohol, cigarettes, etc. when used in excess can indirectly and/or directly affect the people around you and yourself. When you drink too much alcohol, you become more aggressive and violent. When you smoke too much, you damage your health and put other people at risk due to secondhand smoke.
What you do with your body is your own choice, not anyone else's. A lot of things that are legal and taxed are really bad for you.. The interferences of personal choice are for personal gains, power lusts, and more power for big pharmaceutical companies. Drugs aren't necessarily a bad thing when taken experimentally. Everybody should have the right to do that. Don't even get me started on how the laws on drugs have created drugs that are way more harmful than the original one that was made illegal.
You should be in charge of your body more than the government. If you choose to do something harmful that doesn't affect others, that's your loss. If your choices end up harming others, that's when you have a problem. Laws should prioritize protecting its citizens but if YOU and you alone make a choice that affects You, then go for it.