re: Why does the government have the ability to restrict what substances are in my body?

Because certain substances in your body may cause you to do things that harm others.

Now, I don't think the government has any business telling you what you can put into your body while at home alone or in the presence of other consenting adults.

But I also have no problem with the government throwing your arse in jail for a long time if you drive or operated heavy machinery while under the influence of hose things. And if you injure or kill someone while under the influence, I think you should be punished with extreme prejudice.