yes, that seems like what's going on. i think that's why they're both so sick, he's either poisoning them to keep them weak and dependent or giving them drugs for the same reason. i thought it was pretty strange that they both have the exact same illness even though they're not related.
I thought the same too, in ch.3 he thought because of master he came to know the sweetness of Honey, in chapter 7 the author show The master with flower, Masato lick poppy flower on others’ hand and Kenta said something like if you never know the taste of nectar.
And to me, the flower looks like poppy flower, poppy flower is the precursor of some drugs like morphine, heroine etc.
I just reread a few chapters to refresh some things and i actually think the master fed the mc drugs which might b what caused him to b in a haze like phase in ch8 and maybe going thru withdrawal symptoms (idk i need a second opinion lol i might be thinking it wrong) but if he did, fuck him two times over