-Turn demons back into humans, Muzan was able to overcome this, but also triggered the rest of the drug in the process. He was still able to nullify the effect of a drug explicitly meant to turn demons back to humans despite them resisting, which is certainly impressive.
-It physically made Muzan age 50 years every minute, by the end of the fight he had aged nearly 9000 years but was still swinging all the while.
-Prevented his body from dividing or splitting, which would have let him escape the effects of the poisond/drug if done correctly. Muzan could also split himself into thousands of pieces at will and than reform later on. I imagine the ability to literally just split apart would do rather well against Shuten's poison, if Muzan can't naturally overpower it with his absolutely bullshit regeneration.
-And finally, the cellular destruction part of the drug, which was causing basically every cell in his body to die. He was able to ignore this part of the drug until the very tailend of the fight, and even then he was still able to force himself to regenerate to try and overcome Tanjiro at the end. (He was presumably just overpowering this part of the drug through his pure regeneration rate before he began aging as much as he did, and taking as much damage as he did)
All of this together still was not able to actually kill him, all it did was slow him down in tandem with all the Hashira and Demon Slayers rushing him to prevent him from escaping.