@Kampfarsch
we don't need to know beforehands the effect of the drugs,the author showed that ubuyashiki told tamayo to help them kill muzan and join shinobu with the experiments.
in the drug for muzan to return into human tamayo made 3 other effects that conterattack his skills since she saw what hes capable of,and since shes not an idiot,2 effect for weakining him and the last one for the last shot.
the first one, the aging effect,was even foreshadowed with the white hairs,this is the least plausible one.
the second is obvious since we saw the flashback about muzan vs yoriichi.
preventing his escaping trick so an effect for preveting self splitting.
the last one is just basic cell destruction that began to take effect after he was weakened a lot.