My age 26, I have vitamin B12 deficiency, can I take 1000 mcg vitamin B12 supplement?

Are you vegan? How do you know you have a deficiency? If your doctor diagnosed, what did the doctor recommend? Why aren't you following that recommendation? Don't overthink it. If you self-diagnosed, the dosage of 1000 mcg is extremely high and unnecessary. I am more concerned why someone your age has such a critical deficiency. Don't overdoes and waste your money.