I’ve heard you are these days up to your neck engaged in foreign  books of philosophy.
The two possible readings of the same written word : « pharmakon » (medicine or poison) in fact the confusion between the two is the subject of the text .
I think the dialogue in this passage exists simultaneously on three levels. On the first level, the speaker Thamus shows Theuth that if he where to receive his gift, the opposite of understanding might arise as a written rendition of his own word might lead to a false understanding of his intentions.
On the second level The author of the « oral » text speaks against the knowledge that the sophists represent – that of written philosophy. Plato who wrote down Socrates words is also working against Socrates’s ideas, since the master did not believe in writing down his philosophy.
On the third level, the reader (of the original text in Greek), is forced to use his reason (i.e. his « oral » logic), in order to read and make sense of the text. As he has to apply reason to interpret the word as medicine or poison in the proper logical place .
For as you know well the Egyptians had terms which symbolized both something and its opposite. Like the God Atum, which means bot everything and nothing. Just as a comment, I should perhaps note, that they (the Egyptians) differed with Parmenides claiming that also nothing is God, since God contains everything. Despite the fact that by saying nothing doesn’t exist and shouldn’t be discussed, Parmenides in fact raises to the mind the same term – « nothing » – that he wishes to reject.
And so to the Egyptians nothing exists both in the material (or more exactly i-material) sense, and in the sense that God is not only those things that are in time (i. e. that happen), but also all the numerous possibilities that can occur in a moment in time but do not occur (thus being nothing).
Regarding the poison-medicine meaning of a term you are also most familiar with the serpent of brass cult which came from Egypt. As the logic is that medicine changes a given balance in the body and thus excessive amount of it would kill and so act as poison . It is on based on this idea that the Egyptians medicine-men believed the opposite: that a little amount of poison (like a snake’s poison) would act as medicine). And so the cult of the serpent of brass mentioned in our bible [Numbers, 31:9] emerged at the time of Moses under Egyptian influence.
However these two possible meanings result in that when we approach any Egyptian written text for which no longer an oral witness exists we can not know when the word medicine is written if perhaps they meant poison.
Nothing is better in my eyes dear friend after such heavy material than a light prayer. Do join me in a little prayer, for even you that do not believe in all benevolent God is still a Jew under his eyes…
Perhaps you know this prayer:
God please save our souls, God please save our souls, For so great is your name… (« Ana hoshia na, Ana hoshia na, Ki ko gadol shimkha… »)
It is to this superficial kind of knowledge that the king in the passage points to. Since he speaks of reading as a cause of superficial understanding without oral understanding. This claim is further supported by the fact that the king indeed prefers the oral culture, and it is it that holds the solution to these passages.
There is another similar exchange of terms in stories where there is confusion again on the same word – between magician and scapegoat (both written ‘pharmakos’). This when translated to Latin was adapted so that the magician was transformed into a priest and the scapegoat into an donkey .
In the folktale, the priest sings and sees an old woman weeping. He believes it is because she is touched, by his singing. She says he remind her of her donkey (goat) which she lost . This tale emanates from a play on words – as magician (holy man in ancient culture – replaced in the Latin tale by a Christian holy man – a Priest – « Sacerdotem »), reminded the old woman of a scapegoat (both words for magician and scapegoat are similar in Greek), or, as she puts it: her goat that escaped. In the Latin tale goat was exchanged for donkey (Asinum) however the donkey was in Latin a term for scapegoat as was preserved in the proverb « qui asinum non petest, stratum caedit » .
Also, Please note that there is another aspect to the myth mentioned by Socrates. For in the myth, Thamus words seems at first to be correct, as Thamus said the Egyptians acceptance of the Hieroglyphs written language led to their losing (forgetting) their oral ancient language as well as the meaning of their written signs.
However, this myth which rings true in the Greek language where the written word « pahrmakon » has two distinct oral pronunciations and meanings, is lost, once it is comprehended that the dialogue between Thamus and Theuth took place in the lost ancient Egyptian language. For, it is impossible to know for sure (we can only assume so, but not be certain) if the Egyptian word for poison was written the same as the words for medicine.
So it appears that Plato is showing here that myths (Here a Greek myth) are actually superimposition of later ideas on an lost and therefore uncheckable past (Here the ancient Egyptian past). This conclusion is consistent with what has been said elsewhere about Plato’s figure of Socrates – i.e. that he invents false new myths (stories of new gods) as it pleases him .
The fact that the oral myth told in this writing is false negates Thamus’s claim that oral language has an advantage over written language in that it encourages true wisdom and memory. For, as exemplified by the false myth here, oral myths that are transmitted from generation to generation could still be false because they are the invention of « flesh and bones » humans who can make intentional and unintentional mistakes when creating them. Also, those who orally transmit them could transmit these mistakes as truth, for they might remain unnoticed.
The tradition of the confusion between the signifier « pharmakon » received less adaptations in literature than that of the « signified ». That might be because perhaps the signified could transcend language barriers and overcome the decline of Hellenistic culture in the west.
The most famous example perhaps of such an adaptation is Shakespeare’s « Romeo and Juliet », where poison is confused with sleeping medicine (potion). This confusion is possible due to the medical knowledge of that time where pulse readings to show life were not the usual practice but perhaps calling out the person’s name loudly was.
This exchange might also point to the figure of Moses, as he as a kind of magician, performing magical feats to convince Paraoh to release the people of Israel. At the end Paraoh decides to send Israel out of Egypt as a kind of scapegoat to prevent further evil magic from God unto Egypt (however he changes his mind later).