According to WebMD, aconite root contains chemicals that are poisonous and can damage the heart, nerves and muscles. The poison in aconite acts quickly, causing severe side effects, including breathing problems, nausea, sweating, vomiting and weakness. Aconite can be deadly even when applied to the skin rather than being ingested.
Wikipedia points out that symptoms of aconite poisoning occur within one hour at the most from ingesting the herb. If untreated, death usually occurs within six hours of ingestion and can be instantaneous if a large dose is consumed. Aconite poisoning is treated with activated charcoal and atropine plus drugs used for ventricular arrhythmia if the symptom is present.