Injection may be the fastest way when you are trying to get the toxin to a specific location. However, if you are trying to mimic the actual mode of entry in the real world, it might be better to deliver the toxin, for example, via the gastrointestinal system by ingestion or via the respiratory system by inhalation. Thus, the method you choose will depend on the actual mode of entry that you are investigating.

Thank lot my dear Richard A Durst for comment and contribution

Intravenous injection is the fastest. However, the route of administration may determine the fastness of appearance of toxicity signs. Its antidote can also using the same route.