Can Vitamin D3 Be Used Instead Of Vitamin D2?

I recently found out I m deficient in Vit. D (10 ng/mL), & my doctor recommended 50,000 IU of Vit. D2 to be taken once a week for twelve weeks. Research suggests Vit. D3 is the better way to go. It seems D2 is what s readily available at pharmacies, although I talked with a pharmacist who said they had some OTC (50,000 IU Vit. D3), made by Nivagen. I was under the impression you could only get that much by prescription. I m trying to find out where to buy D3 and how do you know if it s made by a reputable company? Any info would be greatly appreciated. Thanks..... Lisa