Forms of the Hebrew word כַשֵּׁף are translated into Greek with equivalent forms of pharmakous: Exodus 7:11, Exodus 22:18, Deuteronomy 18:10, 2 Chronicles 33:6, Isaiah 47:9, Isaiah 47:12, Jeremiah 27:9, Daniel 2:2, Malachi 3:5. As you said, most of these places rule out poisoner from the context. Poisoner simply cannot work in them. The LXX also uses pharmakous in these verses:
- Exodus 9:11 The magicians could not stand before Moses because of the boils, for boils were on the magicians and on all the Egyptians. (for the Hebrew chartim, an occult magician)
- Psalm 58:6 that does not respond to the magicians, or to a skilled snake-charmer. (for lachash and then cheber)
Then three times in the Aramaic sections of Daniel (2:27 - ashaph conjurer, and 5:7 and 5:8 - chartim, occultist). In writings original to Greek, forms of pharmakous are also found.
- Wisdom 12:4 whom you hated for deeds most odious-- Works of witchcraft and impious sacrifices; (NASB)
- Wisdom 18:13 For though they disbelieved at every turn on account of sorceries, at the destruction of the first-born they acknowledged that the people was God's son. (NASB)
- Galatians 5:20 idolatry, sorcery, hostilities, strife, jealousy, outbursts of anger, selfish rivalries, dissensions, factions,
- Revelation 18:23 Even the light from a lamp will never shine in you again! The voices of the bridegroom and his bride will never be heard in you again. For your merchants were the tycoons of the world, because all the nations were deceived by your magic spells!
- Revelation 21:8 But to the cowards, unbelievers, detestable persons, murderers, the sexually immoral, and those who practice magic spells, idol worshipers, and all those who lie, their place will be in the lake that burns with fire and sulfur. That is the second death."
- Revelation 22:15 Outside are the dogs and the sorcerers and the sexually immoral, and the murderers, and the idolaters and everyone who loves and practices falsehood!
The Greek lexicons give these defintions:
- Liddell, Scott, Jones: links it to magic from drugs and specifically states using poisons.
- Friberg: one who prepares and uses drugs for magical purposes or ritual witchcraft
- United Bible Society: sorcerer, one who practices magic or witchcraft
- Lou-Nida: sorcerer
- Thayer: pertaining to magical arts. And points out how it is used for kashaph.
- Lust, Eynikel, Hauspie: mixer of magical potions, sorcerer, magician
- Gingrech: magician
Only one of the major lexicons mentions "poisoner" as a meaning of the Greek word, several point out the drug use, but all agree that it is magical. There was also the cult of Pharmakos which used drugs on their ritual victims.