The "pain in the knees" is an allegory, as the writer explains:
the expression which I use, & which she understands, to mean desire.
Like in the expression "weak in the knees", only so much more that it gets 'physical', like pain?
So it is not a painkiller like aspirin or morphium, but something that "prevents desire".
In modern terms, we're looking not for something medical, but for something magical.
As such lead is indeed an ancient ingredient and even preferred material for magical ritual:
Scholars have debated the possible motivations for using erotic magic, including unrequited love, sexual control of the intended target, financial gain, and social advancement. The love spells used were similar in design around the Mediterranean world, and could be adjusted to different situations, users and intended victims. Recent scholarship has shown that women used curse tablets for erotic magic much more than originally thought, although they were still in a minority.
While the Wikipedia page lists primarily Mathew W. Dickie: "Who practised love-magic in classical antiquity and in the late Roman world?", The Classical Quarterly / Volume 50 / Issue 02 / December 2000, pp 563 - 583 DOI: 10.1093/cq/50.2.563, this paper might be misleading, as
What difference do these two models make to the understanding of women’s use of erotic magic? At the level of historicity they contrast quite vividly. Were prostitutes in fact the predominant female users of erotic spells? What other women used erotic spells, and in what situations? For Faraone, many women used erotic spells, if mostly of the milder philia type. For Dickie, good women shunned magic, but bad women used every kind of spell. The models also differ in their very understanding of women’s ritual acts as expressions of aggression and desire and in relationship to social systems like marriage and the family. For Dickie erotic magic was fundamentally anti-social and predatory, while for Faraone erotic magic included types that were expressly, even anxiously protective of marriage and the family.
–– Kimberly B. Stratton & Dayna S. Kalleres (Eds): "Daughters of Hecate. Women and Magic in the Ancient World", Oxford University Press: Oxford, New York, 2014.
A weakness of the theory presented here might be that while these binding spells might be used as the author of the novel implies: "to prevent desire", they were most often dual-use weapons:
In Roman-era Egypt and Hadrumentum (a city near Carthage) this same early Greek tradition of binding spells yields yet another composite: the so-called binding love spells (philtrokatadesmoi), which aim to prevent the victim from having sex with others and (at the same time) to encourage him or her to have sex with the person performing the spell.
Martinez (1995), who rightly distinguishes (pp. 344–345) “vows of abstinence” (in which someone places himself under a curse until he does something) from “self-curses” (in which someone places himself under a curse after he does something). Both are in fact conditional curses, the former designed to force a desired action, while the latter aims at preventing an unwanted action.
–– Christopher A. Faraone: "Ancient Greek Love Magic", Harvard University Press: Cambridge, London, 1999.
But even in these dual-use scenarios, it still applies that the desire to be prevented is the unwanted one, the socially undesired desire. In this case fitting for "two female lovers" struggling with societal acceptance.
A lead tablet from Attica (DT 68), on the other hand, is clearly set in a context of erotic rivalry. Dated to around 300 BCE, it asks that a woman named Theodora “be unsuccessful (ἀτελής), whenever she is about to converse (διαλέγειν)” with two men, Kallias and Charias, and that “Charias be forgetful of the bed (κοίτης) and child of Theodora.”
–– C. A. Faraone: "Notes on Some Greek Magical Gems in New England", Greek, Roman, and Byzantine Studies 53 (2013) 326–349.