I've been taking phenytoin 300mg and divalproex 500mg once daily for the past 7-10 years - should I be taking calcium, Vitamin D3 and Vitamin B12 supplements, and what dose of a Vitamin D3 60000IU Soft Gelatin Capsule would be right?

Long-term use of anti-seizure medicine can genuinely lower calcium and Vitamin D levels over time, so it's smart to be thinking about this after 7 to 10 years on treatment. Many people in your situation do benefit from calcium, Vitamin D3 and B12 supplementation, but the right dose, including how often you'd need a Vitamin D3 60000IU Soft Gelatin Capsule, depends on your current blood levels. I'd get your Vitamin D, B12 and calcium checked and go over the results with your treating doctor, since they can balance this alongside your seizure medication.