[1] No single word in English captures the play of signification of the ancient Greek word, pharmakon. Derrida traces the meanings assigned to pharmakon in Plato's dialogues: remedy, poison (either the cure or the illness or its cause), philter, drug, recipe, charm, medicine, substance, spell, artificial color, and paint. The word pharmakon is overdetermined, signifying in so many ways that the very notion of signification gets overloaded. A translation problem? Yes and no. In choosing one meaning translators often decide what in Plato's texts remains undecidable. But as indicated above, the problem is inherent in its very principle, situated less in the translation from one language to another, than already within the Greek language itself. And adopted within philosophical discourse, pharmakon does not suddenly become unambiguous, ready and suited for dialectic operations. (In Phaedrus, Socrates tries to distinguish between two kinds of words, the unambiguous - words about which we all agree - and the ambiguous - words about which we are at variance. In Plato, Derrida and Writing, Jasper Neel argues that in fact there are no unambiguous words.) Instead, words like pharmakon threatens the philosophical process, threatens dialectics from within. Plato's text itself is thus already the battlefield of an impossible process of translation.
Plato's systematic and pure reasoning (followed by Platonism and the entire Western tradition of philosophy) experiences great difficulty with this undecidability. It wants to put a stop to this constant shift from one meaning to another and back again. Still, Plato cannot escape the ambiguity of pharmakon either. In Phaedrus, writing is first presented as a useful tool, a beneficial drug (pharmakon). It later proves to be a harmful substance, benumbing to the soul, memory and truth, a poison (pharmakon). In Phaedo, the reverse happens: first, the hemlock is presented to Socrates as a poison (pharmakon). Yet it is transformed, through the effects of the Socratic logos, into a cathartic power (pharmakon), helpful to the soul that it awakens to the truth of eidos.

[2] In Phaedrus, the god Teuth presents writing as a recipe (pharmakon) beneficial for memory to King Thamus. But the King refuses the gift saying that it will produce forgetfulness; it is not a remedy for memory, but for reminding. Writing is a poison (pharmakon) and Teuth has passed a poison off as a remedy. The pharmakon thus produces a play of oppositions: remedy-poison, good-bad, true-false, positive-negative. According to Derrida, this means that far from being governed by these oppositions, the pharmakon (writing) enables the coming into play of oppositions without allowing itself to be fully encompassed by them, without being subsumed under concepts whose contours it draws (cf. Dissemination, p.103).
Writing is an external supplement to internal memory. But even if it is external, it affects memory and touches its very inside. That is the effect of this pharmakon. The pharmakon is 'that dangerous supplement' .

[3] Socrates puts his most effective medicine (pharmakon teleotaton), living knowledge, opposite the other pharmakon, writing. 'Philosophy thus opposes to its other this transmutation of the drug into a remedy, of the poison into counterpoison', says Derrida. This is only possible due to the ambiguity of the pharmakon; it already bears its own opposite within itself. Presenting itself as a poison, it may turn out to be a cure. 'The 'essence' of the pharmakon lies in the way in which, having no stable essence, no 'proper' characteristics, it is not, in any sense (metaphysical, physical, chemical, alchemical) of the word, a substance ... It is rather the prior medium in which differentiation in general is produced' (Dissemination, p.125-6).
This undecidability or doubleness of the pharmakon does not mix two separate elements together. 'If the pharmakon is ambivalent, it is because it constitutes the medium in which opposites are opposed, the movement and the play that links them among themselves, reverses them or makes one side cross over into the other (soul/body, good/evil, inside/outside, memory/forgetfulness, speech/writing, etc.) ... The pharmakon is the movement, the locus, and the play: (the production of) difference. It is the differance of difference' (Dissemination, p.127). In this undecidability, in this non-substance and non-locality, the pharmakon places itself outside the dialectical system and opens a labyrinth or an abyss. This does not turn pharmakon into a transcendental. It is not above the play of delay and difference, rather it is permeated by these. Pharmakon is not the name for the other, but the place where the other is evoked.
When, during translation or exegese, one chooses the word 'pharmakon' to have a single meaning (for example, 'remedy') while the same word also signifies its exact opposite (poison), the choice of only one of the meanings will have the effect of neutralizing the textuality of the text. 'Textuality being constituted by differences and by differences from differences, is by nature absolutely heterogeneous and is constantly composing with the forces that tend to annihilate it' (Dissemination, p.98). The translation of 'remedy' is not, of course, incorrect; but it is incomplete. 'Such an interpretative translation is thus as violent as it is impotent: it destroys the pharmakon but at the same time forbids itself access to it, leaving it untouched in its reserve. The translation by remedy can thus be neither accepted nor simply rejected' (Dissemination, p.99).

[4] (The working of) the word 'pharmakon' as related to the music of John Zorn is detailed on the pages entitled Saprophyte and Zorn's Pharmacy.