The Short Version
The claim is directionally correct but significantly overstated. Potato soap can cause mild side effects such as skin irritation or allergic reactions (particularly in individuals with patatin or latex sensitivities), but the severe harms commonly associated with skin bleaching—mercury poisoning, ochronosis, adrenal suppression—are linked to toxic agents not found in typical potato soap. Additionally, scientific evidence that potato soap actually lightens skin is weak, undermining the premise itself.