In RM2, the Ch&CL's note about "enchanted herbs, breads and poisons price chart" clearly mentions that "if a poison paste is used and a critical is obtained, the foe must make a RR or he receives the effect; if the attack is merely hits (no crit), the poison is gone." and that a "liquid" is "as paste, except it remains effective for 1 hour."
It's in the section no one apparently ever read (or remembers), as I quoted it over and over again when it comes to poison (just as no one seems to remember that, for poisons, RM2 offers two alternatives to the RR method, the luck-based method and the constitution-based method…)
So, yeah, a critical is needed.