Diets and nutritional supplements have been included in neurological studies of epilepsy for many years.  It would be wonderful if the cause of epilepsy or the treatment of epilepsy could be found in something as simple as a nutritional deficiency, but unfortunately that hasn't been the case.   If your dog is being fed a nutritionally balanced diet,  vitamin supplements are unlikely to improve seizure control unless there are underlying factors that interfere with the utilization of nutrients.   However, in most cases, vitamin supplements within reason will do no harm and may improve the overall health of your dog.  Please talk to your veterinarian before adding any supplements to your dog's diet. 

The following is some information about the nutrients that some people recommend for dogs with seizures.  Please note, there is no conclusive evidence that any of these nutrients has an effect on seizures.  Please consult with your veterinarian before adding any supplements to your dog's diet. 

Vitamin B Complex - is a group of water soluble vitamins that work together.  An excess of one of the B vitamins can cause a deficiency of another, therefore, when B vitamins are supplemented they should always be given together as a complex.

Vitamin B6 deficiency has been implicated in certain  rare types of human epilepsies such as Pyridoxine-Dependent Seizures.  Pyridoxine-Dependent Seizures is a rare genetic cause of intractable seizures in neonates, infants, and young children.  I was unable to find any studies on the use of vitamin B 6 to treat dogs with epilepsy.

B vitamins are water soluble and excesses are excreted in the urine making overdoses harmless with two exceptions of extreme excess: 

Extreme excess of vitamin B6 can cause ataxia (alterations in balance) and sensory neuropathy (changes in sensations due to nerve injury) and possibly liver disease. 

Vitamin B3 used in extreme excess can cause gastrointestinal distress and mild to severe liver damage.

VITAMIN C - Although most researchers believe that dogs can synthesize vitamin C internally and supplementing would be wasteful, some believe that supplementing with vitamin C is beneficial.  Vitamin C  has many beneficial effects including increased collagen synthesis, carnitine synthesis, protection against free radical damage, and nitrate scavenging.  There are no reliable reports indicating that vitamin C is useful in the treatment of seizures in humans or dogs.  

Currently vitamin C is not recognized by the AAFCO as a required nutrient and therefore it is not included in most dog foods (except in small amounts as a preservative) or supplements formulated for dogs.

Vitamin C is a water soluble vitamin and excess amounts would be excreted in the urine, so a little extra should not hurt.   There are no known adverse effects associated with over supplementing, however, some researchers believe that extreme excesses may alter the pH balance of the kidneys and a recent study in people indicated that high dose vitamin C supplementation is sometimes associated with an increased risk for heart disease.

VITAMIN E - is a scavenger of free radicals and a powerful antioxidant.  Significant research has been done on the relationship between antioxidants and seizures in humans and rats.  Unfortunately these published studies have shown mixed results.  It does appear that the use of vitamin E may reduce brain damage from the rise in lipid peroxides and free radicals that seizures cause.

None of these studies reported any adverse effects to patients taking vitamin E supplements.  The lack of adverse effects, coupled with some studies that indicate that vitamin E may be beneficial at controlling some types of seizures and preventing brain damage from seizures make this a supplement that may be worth discussing with your veterinarian.

Taurine - is an amino acid and possibly a neurotransmitter.  Dogs and most other mammals (cats are an exception) can synthesize taurine from other amino acids in the diet.

There are a number of reports that taurine may have mild anti-convulsant effects in people.  There are also a few published studies on rats that show similar mild effects, however, one study showed that rats on a taurine deficient diet actually had less seizures.  Prior to supplementing with taurine, you might want to have the taurine level measured to determine if it's low.

Adverse effects have been reported in humans from over-supplementing with other amino acids, however, I found no reports on taurine supplementation.

Minerals

Calcium - A low calcium level can cause seizures in dogs however, hypocalcemia is usually caused by an underlying disease such as hypoparathyroidism, renal failure or pancreatitis.  Calcium should never be supplemented in dogs without proper supervision from your vet as calcium and phosphorus imbalances can cause significant health problems in dogs.

Magnesium - Magnesium is needed for more than 300 biochemical reactions in the body. It helps maintain normal muscle and nerve function, keeps heart rhythm steady, and bones strong. It is also involved in energy metabolism and protein synthesis.  There have been some studies that indicate that magnesium may have anti-convulsant properties in kindled rats, but the available information is inconclusive.  

Signs of excess magnesium can be similar to magnesium deficiency and include mental status changes, nausea, diarrhea, appetite loss, muscle weakness, difficulty breathing, extremely low blood pressure, and irregular heartbeat.

Selenium - Is an essential trace mineral that is important for the normal functioning of the heart,  immune system and thyroid gland.  Studies have shown that selenium may help humans with arthritis, heart disease and one study indicated that a selenium deficiency may have contributed to seizures in two infants.

Side effects from overdoses of Selenium are rare, however in extreme excess Selenium is toxic causing tissue damage and hepatitis.

Zinc - Is a trace element that is vital in the metabolism of B-complex and other vitamins.  A deficiency will show up as slow or retarded growth, skin problems or diabetes.

According to Phyllis Balch's most recent book, "Herbal Prescriptions for Common Health Problems", recent research with animals has indicated that high levels of zinc in the brain, lower the threshold at which seizures are triggered, therefore, until further studies are completed, Zinc should not be included in supplements for epileptic dogs.

Zinc toxicity has been seen in both acute and chronic forms.  Zinc can be  extremely toxic to dogs and may cause multiple organ failure, seizures and cardiopulmonary arrest.