Almost every psychological drug is either the same as, or similar to, some substance in our brain. They generally work by increasing the supply of that molecule in our brain. However these drugs are in very low concentration in our bodies. If you had drugs outside your body at the same concentrations that they were in your body, it probably wouldn't be illegal. Or at least nobody would be able to prosecute you. That's cause it would take a real lot of effort to even figure out the drug was present, and it couldn't be used to affect someone mentally to a noticeable degree.

I agree that it's stupid that drugs are illegal. The war on drugs has done much more harm than good. But trying to say it's wrong that these drugs are illegal because they exist naturally in our bodies is intellectually dishonest. You are propping up a strawman argument to punch it down.