It does not take a lot for dogs to OD on drugs and related substances. They are sensitive to narcotics and the effects of illegal substances, and do not have the tolerance that humans have or could develop for these. They usually ingest only basic foods and their systems are not especially immune to even the mildest dosages.
Overdosing thus will happen when they ingest just a small amount of drugs like heroin or raw opium. K9 opioid overdose happens in the line of duty, because the dogs that often come into contact with opioids are those tasked to sniff them out. K9 units are often the most effective detectives in discovering contraband of this sort.
Breeds of dogs used for the units in question are usually working breeds, big and efficient and very intelligent. Their talent for sniffing is basic to the specie, but for trained sniffers of drugs, it is enhanced by training. They know the smell of their opioids but may not be able to help themselves when it comes to ingesting them.
That is why the process for these units is usually for the canines to sniff out where the contraband is located. And then the human personnel can do the handling of the stuff. However, there might be times when the expert police handler is not able to prevent his charge from happening on a package of heroin and bite and then lick its contents.
There are two things that could happen from there, one is that instantaneous highs are achieved. Second is that these highs will usually result in overdose, and the need for emergency response in this case will be dire, although the handler himself or herself may have the Naloxone antidote at hand for just such emergencies.
Dogs often forget their training when they are faced with systemic narcotization. This means they are less in control of their reactions that humans do when they ingest these substances. The Naloxone is often administered after a mild sedative shot is given to the hardworking animals.
This often works well enough, although no one ever knows what the effects are or how much a dosage can actually kill a valuable and highly trained canine. There have been cases in which the dogs were killed outright. These have made the handlers and the training programs for K9 programs more aware and with it in methods and response times.
It may be only a matter of seconds before the canine system stops and goes into the death spiral. Or it could take minutes, but the difference does not really matter, since and overdose could permanently damage a trained dog. Again, the canine system is sensitive to drugs, and a few granules could do the trick for them.
That is why operational methods in place today protect these animal detectives from actually coming in contact with the substances they are tasked to locate or discover. This is not that hard to do, and prevention is always the better part of valor. Dogs do dedicated work all the time and do not require salaries and so they are protected from any adverse effects of their work.