You should contact your vet after you first notice that your dog is having seizures. They will do a full physical exam and test bloodwork to determine if there is any obvious underlying cause that could be causing the seizures.
Your vet may or may not choose to prescribe medication. They may also choose to monitor the situation and see if a second seizure occurs, as medications for seizures are prescribed on a case-by-case basis. The length of the seizure, the frequency and the general demeanour of the dog after and before the seizure are the factors that determine when medications are used.
Dogs that have seizures are often referred to a vet neurologist for more advanced workup like an MRI or cerebrospinal fluid analysis. This is the best way to identify the underlying cause of the seizure and target the treatment specifically. You should contact your vet whenever your dog has a seizure to discuss the best option for their condition, even if it’s just to let your vet know how frequently your dog’s seizures are happening.
However, if your dog has a prolonged seizure (lasting greater than 5 mins or having recurrent seizures without regaining full consciousness in between) this is called ‘status epilepticus’. A “cluster seizure” is where a dog has more than one seizure within a 24 hour period.
Both status epilepticus and cluster seizures are medical emergencies, and you should take your dog to a vet immediately. Seizures themselves can cause damage to the brain so prolonged seizure activity needs to be stopped when possible.
A 24-hour emergency veterinary hospital is the best place for a dog who is having cluster seizures or status epilepticus. Get them in the car, try to pad them up for some protection, but ultimately getting them to the vet is the safest thing you can do. Always be careful around your dog’s head as they can easily unknowingly bite or clamp their teeth during seizures.