There's a synthetic opioid called MPPP, which, if inappropriately synthesized (IIRC, using too much heat in one step), yields MPTP, which is non-toxic in and of itself, but has the ability to penetrate the blood-brain barrier, where it is then metabolized into MPP+, which is potently neurotoxic to the dopaminergic neurons of the Substantia Nigra, reliably producing a Parkinsonism in those exposed to it (from wikipedia):

>The chloride salt of MPP+ found use in the 1970s as an herbicide under the common name cyperquat.[4][3] Though no longer in use as an herbicide, cyperquat's closely related structural analog paraquat still finds widespread usage, raising some safety concerns.

EDIT: the neurotoxicity of MPTP was discovered after a number of heroin addicts developed a sudden, irreversible Parkinsonism after injecting bad batches: https://archive.org/details/TheCaseoftheFrozenAddict

The doctor featured in that NOVA episdoe summarizes the history of MPTP and its relevance to Parkinson's research and epidemiology here: https://pmc.ncbi.nlm.nih.gov/articles/PMC5345642/

> Over the last two centuries the pendulum of opinion has swung widely as to whether the cause of PD was due to genetics or environment causes [69]. While MPTP has not yet been found in the native environment, beginning in the 1980s the pendulum swung dramatically in the direction of the environmental hypothesis, spurred not only by the observation that a simple pyridine (MPTP) could induce so many of the features of PD, but also the striking similarity between its toxic metabolite, MPP+ and paraquat (differing only by one methyl group) [70], an herbicide that is used worldwide. Since that time, a large number of studies have shown pesticide exposure is a risk factor for PD [71]. Interesting, this risk is enhanced by the presence of certain genetic variants [72], consistent to the adage that “genetics load the gun, but environment pulls the trigger”.