Justin Bieber partied with his boys this week ... and there are photos that appear to show a large bottle of codeine at the shindig -- aka sizzurp.

The photos show Justin, Lil Za and Lil Twist (we're told both of whom have smoked pot with Bieber) at a table rolling up blunts. Also on the table -- two sets of double cups.

Sources connected to Justin adamantly deny he uses the drug. Based on multiple recent photos, however, it's pretty obvious he smokes pot. In the pics, we don't see anyone present actually consuming the drug.