Symptoms range from eye irritation and vomiting to internal bleeding and organ damage.
Other items stolen included tools and other miscellaneous items.
The theft comes after a series of burglaries and attempted burglaries in Hawera during the past few days.
One dental clinic was targeted twice in a few days and a computer was stolen. There was also an attempted break-in at a local dental surgery.
Detective Guy Jackson said it was not clear if the other incidents were linked.
"It is not clear if these incidents are linked to the Beach Road burglaries but the medical nature of their work makes us concerned that they are being targeted in the hope of finding drugs. If so it is a dangerous game that the thieves are playing."
Police urgently want to find the Pentobarb 300, if anyone knows who is responsible for the theft or who has the drug, they are asked to contact police immediately.
Anyone with relevant information is asked to contact Hawera Police on 06 278 0270. Or information can be provided anonymously via Crimestoppers on 0800 555111. In the event of poisoning, contact the National Poisons Centre on 0800 764 766 for advice.