Many clinicians hold that good pain relief with NSAIDs of patients with flank pain should further your suspicion of nephrolithiasis as the etiology. This effect is reportedly due to a decrease in  ureterospasm and decreased in GFR leading to reduced renal capsular pressure in the obstructed kidney.  Still, many prior trials have had mixed results in proving equivalency or superiority or NSAIDs to opiates in nephrolithiasis, mainly due to problems in study design. But now, in a study published in nothing less than The Lancet, there is a large well-designed study to justify your decision to reserve opiates only for the more unrelenting cases of renal colic.

1,316 patients with confirmed ureteric calculi were enrolled in a randomized double blind controlled trial using intention to treat analysis. Participants were divided into groups receiving IV morphine vs. IV diclofenac vs. IV paracetamol. The primary outcome studied was achieving at least a 50% reduction in initial pain score. This score reduction was achieved in 68% patients in the diclofenac group, 66% in the paracetamol group, and 61% in the morphine  group.

Read more:
“Delivering safe and effective analgesia for management of renal colic in the emergency department: a double-blind, multigroup, randomised controlled trial”
https://www.ncbi.nlm.nih.gov/pubmed/26993881