Also an apex. I think the patient means there are so many patients who could go in either direction (at the crossroads): either recover or go down the tube (become hopelessly ill). They have potential but the doctors are just keeping them medicated and not helping them recover.

cusp
n. prominence; crown of tooth; sharp point; Astrology, cross-over point between two signs. cuspal, adj, cuspate, adj, having cusps. cuspid, n. canine tooth. cuspidal, a. cuspidate, adj, coming to a point.
http://www.tiscali.co.uk/reference/dictionaries/difficultwor...

cusp = the dividing line between two very different things eg on the cusp of adulthood

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Note added at 4 hrs 27 mins (2004-12-29 04:57:27 GMT)
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cusp = 1 a pointed end; peak 2 any of the elevations on the chewing surface of a tooth 3 any triangular fold of a heart valve 4 either horn of a crescent, as of the moon 5 Geom a corner point formed by two tangent branches of a curve.

-> 1 and 5 indicate the idea of a sharp turning point, going over the edge

perhaps you can also say \'on the verge\' (of mental illness)

This is the meaning inferred from the conversation. the speaker says that they are human beings, and as they are medicaterd they ask for respect whem getting medicated as some of them at on the cusp (meaning that some of them are highly productive and they don't want to be destroyed by medications) That's what I can deduct from the small paragraph.