Calcirol (cholecalciferol) 6 lakh IU is a thick, oily depot injection given deep IM into the buttock. What you're describing - pain at the injection site spreading down the whole leg - is a recognized issue with gluteal injections, and it's worth taking seriously rather than just waiting it out. There are two main possibilities:

1. Simple post-injection muscle soreness (most common, benign) High-dose vitamin D depot injections are viscous/oily and often cause local muscle irritation, mild inflammation, or a small sterile reaction at the injection site. This typically causes:

2. Sciatic nerve irritation/injury (less common, but important to rule out) The gluteal region overlies the sciatic nerve, and if the injection is placed even slightly off from the correct site (upper-outer quadrant of the buttock), the needle can irritate or directly injure the nerve. This is a well-documented complication of gluteal intramuscular injections - textbooks note that "serious sciatic nerve injury can result from improperly performed injections into the buttocks," with older age and low muscle mass being risk factors (Andrews' Diseases of the Skin, p. 79; Campbell's Operative Orthopaedics). Pain that radiates the entire length of the leg (not just local soreness) raises this possibility, especially if you also notice:

I can't examine you, so I can't tell you which of these is happening - but given that the pain has spread down the entire leg (not just stayed local), I'd lean toward getting it checked by your doctor within the next day rather than waiting it out, just to rule out nerve involvement. Please don't ignore any weakness or numbness if it develops.

Is the pain a dull soreness, or does it feel sharp/shooting/burning down the leg? And have you noticed any numbness, tingling, or weakness in your foot or toes?