In nature, the rare disease botulism occurs when ingested food is contaminated by large quantities of botulinum toxin, which is a protein produced by the bacteria Clostridium botulinum. Scientists have been able to isolate and purify extremely small quantities of botulinum toxin that can be used clinically without fear of the disease botulism occurring.

When botulinum toxin is injected into individual muscles, partial weakness (usually lasting 3 to 6 months) results. Botulinum toxin treatment can be useful in treating neurological diseases characterized by overactive muscle activity limited to a few muscles.

These diseases include:
1. Focal dystonias involving the neck, eyelids, chewing muscles, vocal cords and arm muscles.

2. Hemifacial spasms

3. Tremors of the arms and head

4. Muscle spasticity in the arms or legs resulting from strokes, multiple sclerosis, and other causes.

Botulinum toxin is usually delivered via a hollow electromyography (EMG) needle which helps localize the overactive muscles. It take 2 to 3 days before any effect is noticed and maximal weakness occurs 2 to 3 weeks after injections. Because only temporary weakness is produced repeat injections are required every 3 to 6 months if clinical benefits are to be maintained.