In general, fennel is a very safe herb for your dog. Fennel seeds are often used by canine herbalists to relieve gas and pain in the abdomen.
If your dog has gas due to inappropriate eating, a cooled fennel tea can work wonders. Add one teaspoon of fennel seeds to 250ml of boiling water, soak until cool. Give your dog two to four tablespoons per 10 kg body weight.