The side effects of most pain killers can also be extremely unpleasant. You'll have more time, but you'll be miserable or marginally conscious for most of it.
And they don't always work. Look up intractable pain.
In case of opiates, what side-effects are you thinking of? Constipation is an easy fix. Assuming proper use and dosage, opiates are way safer than NSAIDs.