Are there any studies specifically on long-term dihydrocodeine harms?

The short version: there are no studies here that test long-term dihydrocodeine harms directly. The evidence is about other opioids and other topics entirely.

The retrieval surfaced one safety review of dihydrocodeine from 2015 [1], but it covers general adverse effects and dosing limits, not long-term outcomes. It reports that dihydrocodeine contributed to 6.8% of opioid-related deaths in the UK over a decade [1], which is a population signal, not a controlled study of chronic use. A 2017 trial in Chest tested dihydrocodeine's effect on breathing during a single dose in chronic cough patients [2] — that is an acute effect, not long-term harm. The other three records are about bariatric surgery, kidney transplant tolerance, and phenylketonuria treatment [3, 4, 5]. They have nothing to do with dihydrocodeine.

So the evidence base for your question is essentially absent from this retrieval. No study here followed people on dihydrocodeine for months or years and measured harms. No study compared long-term users to non-users. The safety review [1] is the closest thing, and it is a narrative summary, not a prospective cohort or trial. The acute respiratory depression finding [2] is relevant to overdose risk but tells you nothing about dependence, cognitive decline, constipation, or hyperalgesia over time.

What you can say from the opioid class literature outside this retrieval is that long-term opioid use carries well-documented risks. But that is general knowledge, not evidence specific to dihydrocodeine. The question you asked — are there studies specifically on long-term dihydrocodeine harms — is answered by this retrieval with a clear no.

My call: the evidence here does not include a single study on long-term dihydrocodeine harms. Confidence: high that this retrieval contains none.