Veterinarians in Colorado have seen a recent spike in dogs suffering illness from marijuana ingestion. Colorado is one of 17 states where medical cannabis is legal, leading to increased availability and risk to pets. Whereas previously vets were dealing with as little as 3-4 cases of stoned dogs a year, they are now handling up to 5 a week. The majority of cases are the result of pets accidentally getting into their owner's pot-based food products, though some incidents were derived from people intentionally getting their dogs high.
Unfortunately, the effects of weed are quite different dogs than they are for people. From CBS:
Most of the time the dogs will end up showing symptoms such as staggering, acting lethargic, vomiting, and being overly sensitive to sound and light. Sometimes they fall into a coma. It's the doggie equivalent of a "bad trip." After treatment most are back to normal within 24 hours.
There have even been some rare incidents in which a dog has died from pot ingestion. To protect your animals, vets recommend treating your weed like you would any other prescription medicine — put it in a safe place that's inaccessible by pets and children.
Look at it this way: the less weed digested by your dog, the more you get to smoke yourself. Everybody wins.