There's been a couple of threads on this. Essentially, the mechanism of death in chloroquine is via intense cardiac trauma, and there have been incidences of seizures, so I would be surprised if it didnt hurt without appropriate sedation. The lethality of its toxicology profile has been verified in the clinical literature, but theres a trade off for that.
This is why the end of life advocates recommend a rather heavy intake of benzodiazepines, or sedatives like ambien, to be taken in conjunction. I'd also be tempted to add an anti convulsant to the mix, but I am no pharmacist.