The toxins responsible for botulism are proteins that inhibit the release of acetylcholine, which in turn can cause muscle paralysis. In extreme cases the paralysis can involve the respiratory muscles, and people can die of respiratory failure. This mechanisms of action indicates that the toxins are

Poisonous substance.

Explanation:

This mechanisms of action indicates that the toxins are the poisonous substance which attacks on the respiratory system of the body that leads to the respiratory failure. Acetylcholine is the main neurotransmitter of the parasympathetic nervous system which is the part of the autonomic nervous system that contracts smooth muscles so inhibition of the release of Acetylcholine is the main cause of paralysis.

a physical change is being made.