We report a case of mild cannabinoid poisoning in a preschool child, after 3-week ingestion of hemp seed oil prescribed by his pediatrician to strengthen his immune system. The patient presented neurological symptoms that disappeared after intravenous hydration. A possible mild withdrawal syndrome was reported after discharge. The main metabolite of Δ9-tetrahydrocannabinol was detected in urine, and very low concentration of Δ9-tetrahydrocannabinol was detected in the ingested product. This is, as far as we know, the first report of cannabinoid poisoning after medical prescription of hemp seed oil in a preschool child.