Sounds like it comes with an increased risk of stroke and cardiological issues, psychomotor agitation, anxiety, aggression, possibly even psychosis. Humans have noradrenaline regulation for a reason.
We'll have to see. There are a lot of natural and experimental painkillers that aren't used either because of side effects or more often because they just don't work consistently. It just so happens that so far the opioids are the only ones that can handle extreme pain. It's a hard problem to solve
Does the article not consider those to be serious side effects then? What is considered serious?
In this case, I believe ”serious side effects” is referring to opioid addiction and physical dependency.