matter of dosage, at least partly

砒霜，水银，雄黄有毒: if you cut down the dose to small enough, they become non-poisonous, or even therapeutic (for example, arsenic cures some leukemia)

花生，海鲜使人过敏: here dosage-response is not a linear quantitative relationship. As long as the allergen reaches the threshold level, there will be the hypersensitivity that is triggered by the allergen, but driven by the host's IMMUNE SYSTEM.