When used properly drain cleaner is a safe way to clean and clear your clogged drains.
But yes drain cleaner can be dangerous to your health if you're not using it correctly.
Drain cleaner can be bad to breathe in so you want to have ventilation when using it and if you start feeling sick from it make sure get outside in the fresh air.
If drain cleaner makes contact with the skin be sure to immediately wash it off and wear gloves when working with drain cleaner.
Drain cleaner can burn the skin and cause rashes when it's in contact with the skin.
Also it can be dangerous to get drain cleaner in your eyes as well so you should wear safety goggles when using drain cleaner.
Drain cleaner should always be kept out of reach of small children or any children who may be young enough to want to mess with it.
If ingested the drain cleaner can be deadly.