My guess would be that he has immunity to poison. A hundred poison immunity seems to be a thing in murim world, according to many murim webtoons I have read.
Spoiler...??? Possibly?
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From my understanding, (bear with me, it has been a while) it wiñl be due to the flame entity he consumed for the dwarves. It will burn away the poison.
Not only that. He remembers how to make the poisons and antidotes from the Murim. Remember he left that poison in that Duke from Chronos house and he took it to the Poison Tower. The Poison Tower could not figure out how to neutralize Roman's poison.
That's cool actually.
Let me guess, Romans normal training makes him immune to poison or he knows the "expel toxins from body" technique