Formaldehyde is not an artificial preservative. It is also produced by the human body and we require small amounts of it to live. It is a product in the important process of the methane cycle.
Outside of niche research chemicals, one of the most dangerous psychoactive drugs is datura, also known as deadly nightshade. It's a delierient in the same class as PCP. It's commonly said that using one seed's worth will cause you to black out and wake up in either the hospital or jail, if you wake up at all, and it's fully legal to grow in your windowsill.
"Natural" does not mean "good for you".