Its sinful because Gods Word tell us in Galatians 5:19 Now the works of the flesh are evident, which are: adultery, fornication, uncleanness, lewdness, 20 idolatry, sorcery, hatred, contentions, jealousies, outbursts of wrath, selfish ambitions, dissensions, heresies, 21 envy, murders, drunkenness, revelries, and the like; of which I tell you beforehand, just as I also told you in time past, that those who practice such things will not inherit the kingdom of God.

"sorcery"
pharmakeia: the use or the administering of drugs

so really, we are not leaning on The Word, and God as our healer and provider...

Chemotherapy, I agree try what we need, but as God permitts. Would not give a kid heroine just because it would make them enjoy it for a moment.

It goes back to the heart, people always struggle with money, beer, weed, sex outside of marriage because its something so common today that its almost impossible to live without. But with God all things are possible... 2 Peter 2:19 also.