PARASITE open thread!

The book mentions that a few of the times that Sal got life-threateningly ill was after she took anti-parasitics*, so evidently there was some attempt (perhaps not by Symbogen) to follow the normal care instructions. Afterward, even doctors not affiliated with Symbogen probably would tread with caution, the same way you would with any patient that has a severely bad reaction to a wide class of drugs.

What might seem more suspect is that the Intestinal Bodyguard is thought to only able to live for 2 years; even if Sal couldn't take the normal anti-parasitics to clear things out, her non-Symbogen doctors would expect her symbiote to die on its own eventually and for her to need a new one. (Then again, Symbogen would be in the position to slip her a placebo, as you say.)

* Which was one reason that she was willing to put up with Symbogen: when you have scary unknown medical problems, having top-of-the-line health care for free probably seems like a good deal AND Symbogen could use it as a 'look you really should work for us, so if this happens again, we can immediately treat you'.