The change drug was a drug invented by Cedric Mathews that made tame creatures wild and aggressive creatures docile. He planned to conquer the world with it.

Mathews planted the drug into the feed of several animals at the zoo Arkwood Private School visited. The Third Doctor and the Brigadier investigated and traced it back to the school. Mathews then planted it in the drinks of Form Four which sent them on a rampage to Arkwood. The Doctor counteracted the effects of the drug with his own gas. (COMIC: The Arkwood Experiments)

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