The starting agent, buprenophine, was taken from MacFarlan Smith in Three Trees Road, Newbie, Annan, sometime in the early hours of Monday.
A thief is believed to have got past the security fence at the site, entered a warehouse and stolen about 22kg of the substance.
It can be used to manufacture pharmaceutical intermediates and was contained in a brown plastic keg.
Detective Inspector Dean Little said: "At this stage I would like to highlight to the person responsible for taking this compound to be aware that this is a toxic material and not to touch, ingest or consider supplying the material to anyone else.
"Consumption of the material is likely to result in significant injury.
"I am keen to recover this material as soon as possible and would appeal to anyone who may know where it is to make contact with police as a matter of urgency."