My poodle keeps gagging and vomiting a foamy substance,What can cause this to happen?

Poisoning usually, get to a vet ASAP.

Many dogs vomit a yellowish foam slime (bile) when their stomachs are empty. I changed my dog from one meal a day to TWO and had great success with reducing the occurrences.

I would say get to a vet also. My pomeranian did that once, she was foaming, vomiting and wouldn't drink her water. What we were told id that is fairly common in small breeds and thankfully she was OK but it is better to be safe than sorry.

Definitely get to the vet! I had the very same problem recently with my beagle/lab mix puppy, and the vet told me that it was possible that it was caused by allergies, internal parasites or even just stress. One way or the other, a visit to the vet is really the best option.

Don't let your dog eat leaves, plants, grass, bushes... It makes them throw up and gag in the process.