Matthew Mindler killed himself with an easily accessible substance he picked up online … and his mother wants everyone to know about the extreme danger of the compound.

The Lancaster County Medical Examiner’s Office tells TMZ … the former child star’s cause of death was sodium nitrate toxicity. According to the M.E., the OD was intentional and as such his death was ruled a suicide.

If you’re unfamiliar … sodium nitrate is an oxidizing agent — one often used to preserve foods, like meat — that causes hypotension and limits the flow of oxygen in the body, which can result in death.