What are some of the possible effects of intravenous botulinum toxin?

Death: Although the effect is dose and time dependent but still, you don't want it in your blood stream to stop vital muscles from working or making them weak.

Many and serious: Botulinum toxins should never be injected intravenous. The side effects will be related to where the medication gets to: if it gets to swallowing muscles then swallowing will be a problem, if breathing muscles then breathing will be difficult. In high enough doses it can result in death. If recognized promptly supportive care can be provided until the effect wears off.