I've heard that the main reason they can be dangerous is because they can contain way too much.
replySupplements are pretty much unregulated, and can have some fairly scary ingredients. The regulators generally don't mess with them, until someone (usually multiple someones) has a problem.
replyLots of dodgy nutritional supplements out there. Look for NSF Certified Safe for Sport products to be sure of what you're putting in your body.
replyhttps://www.nsf.org/knowledge-library/importance-nsf-certifi...
And buy direct from the manufacturer. If you order through a reseller like Amazon then there's always a risk of getting counterfeit products if someone tampered with the supply chain.