Updates to PIP assessment guidance include amendments to part 1 (the assessment process) on assessing pain (para 1.6.35)
HPs should be mindful that the level of analgesia used does not necessarily correlate with the level of pain.
GPs are encouraged to avoid prescribing strong painkillers for long-term pain as the harms usually outweigh the benefits and there could also be specific reasons why painkillers are not prescribed, for example intolerance, or the use of other methods of pain relief
Although it’s an ESA decision, Judge Poynter in  UKUT 446 (AAC)
Whether absence of treatment is an adequate basis from which to infer existence of function also considers the issue.