Bubs, our Border terrier cross is epileptic. as above, remain calm and soothe the dog [stroking/lots of gentle fuss] for when they recover.
Take to the vet in the morning. Our dog has been on medication for over 10yrs and the fits have dwindled into nearly zero. (2 x tablets per day, but the vet will have to find the correct dosage)
Also try a natural dry food. We use James wellbeloved as the mixture does the dogs digestion good which the vet recommended.
Our dog is on Phenobarbatil {not sure if spelt correctly] And costs around £25 per month for the tablets.