To find out the prophylactic role of single dose of vitamin K in prevention of antibiotic induced hypoprothrombinemia. This is comparative cross sectional study included critically ill children in age bracket 4 months to 11 years, admitted to a tertiary care hospital in Afghanistan, likely to receive prolonged antibiotic therapy. 100 children, 50 in each group (A & B) were enrolled within the study. Patient allocation was done on alternate basis. A children received prophylactic vitamin K while B didn't. Baseline coagulation studies and other investigations were wiped out all children. Coagulation studies were repeated on day 10 and day 15 of antibiotic therapy and in between if required clinically. Children who developed deranged INR got therapeutic vitamin K . If deranged INR returns to normal at 11 hour of vitamin K administration then it indirectly confirms vitamin K deficiency. Analysis was done by fisher's t test and chi square test. In children on prolonged antibiotic therapy, vitamin K deficiency was a standard problem (17 %). it had been common in male sex, severe grade of protein energy malnutrition (PEM), Nmethylthiotetrazole (NMTT) group containing antibiotics use and duration of antibiotic quite 9 days. it had been same in children whether or not they received or didn't receive prophylactic vitamin K on day 1 of antibiotic therapy (95 % CI; p value 0.77). Vitamin K deficiency is common problem in patients on prolonged antibiotic therapy. there's no role of single dose of prophylactic vitamin K in preventing antibiotic induced hypoprothrombinemia.