Select statement with substring function

Question

i have textbox for my user to type first 3 or 4   or any number of characters to seach for full name of drug  from sql table  named grugs 

i used Lenght function to find no of characters typed  so i can use this number with substring function to display all drug names started with enterd characthers 

 

int i = textBox1.Text.Lenght; 

select * from drugs where Substring(drg_name,0, i ) =  textBox1.Text 

 

but this gives me Error says Invalid Column name i  

 

any Idea smart pepole ?