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Depends on the painkillers. There's risks with every drug.
There's some evidence that higher-doses of NSAIDs are linked to heart issues in endurance atheletes (as well as the more common gastro issues) and you can do shit like OD on paracetamol (death is pretty permanent damage).
Then there's the simple risk of overuse injury because you'd removed an idicator of pain that would've otherwise made you stop sooner. It can be hard to tell without a lot of experience (of going too far) where the line is for overuse injuries.
I remember caning ibuprofen in TransAm and getting massive cankles due to fluid retention but by the time I was doing later TCRs I was riding 4000k without taking anything.
I've always been wary of painkillers because of the risk of unknowingly doing permanent damage. Is that really true or something I've made up?