Hello, Thanks for asking your query at DoctorSpring.com.
The Vitamin D level measurement is bit less for you. 14 nd/dl will come under a mild to moderate deficiency. The prescribed dose of - vit d 300000 iu im each 3 weeks is rather higher dose, but not very unsafe. This is will be the desecration of your treating Doctor, yet its better to start at a lower dose since you do not have any major deficiency.
There are two options Vitamin D3 (cholecalciferol) is available in 400, 800, 1000, 2000, 5000, 10,000, and 50,000 unit capsules. It is available in some countries as an intramuscular injection, which can be extremely painful.Vitamin D2 (ergocalciferol) is available for oral use in 400 and 50,000 unit capsules or in a liquid form (8000 unit/mL [200 mcg/mL]).Both are available in injection forms also.
So I would recommend you to discuss with the possibility of starting with a lower dose or using a oral capsule.
Hope this helps.
Please feel free to ask any followup questions via email itself.
PS : Reg this "chronic back pain for 2 years with burning sensation in my foot the pain increases by standin , walking and prolong sitting also some degree muscle weakness of lower limbs" I hope you have been evaluated by a neurologist, if not - this needs evaluation.