If you suspect poisoning you must take your dog to the vet ASAP. If it is not poisoning it could be Atopy, or food allergy dermatitis, which should also be addressed by a veterinarian. Allergies are something that a dog will have for life. There is no cure, but there are many treatments. Treatments include antihistamines, steroids, drugs like Atopica, hypoallergenic foods, desensitization therapy, and more.
Occasionally, constant licking of the legs can be a behavioral abnormality due to boredom or a compulsive behavior. This requires sedation or behavioral modification medication such as Prozac.
My Mini Poodle used to lick her paws & legs till it almost became an obcession with her...it was not. It drove me crazy though & I worried alot. I took her to her Vet.
It turned out to be allergies. To what, I'll never really know. Probably pollen & grass. It got worse in the springtime & summer. My Vet put her on a partial steroid..Temoral P & an antiinflamatory injection..dexamethasone, along with an antibiotic..Orbax. It all worked well but the Steroid made her too sluggish & sleepy. I stopped it after 2 weeks.
I must give them to her now & than but not for long. NO MORE LICKING.
Your dog will get better. Keep the faith & good luck.