It is permissible to warn people that a product is dangerous – that is, if the warning is truly warranted. Countless items are potentially dangerous if misused, and many processed foods are unhealthy if consumed in large quantities; in such cases, labeling the item “dangerous” is wrong. Labeling an item as unsafe is justified only if such a declaration is considered reasonable by normal standards. Furthermore, if the information is not known firsthand, this must be mentioned when relating the information.
If the manufacturer is Jewish, he must be spoken to first. Failure to fulfill the above conditions would render the statement loshon hora, as it would cause harm to the manufacturer, as well as to the consumer who will needlessly avoid the product.
Sefer Chofetz Chaim, 15 Kislev, Page 198