What are the risks of taking promethazine hydrochloride with suboxone?

The risks reported are moderate on the Interactions checker, but would however recommend that you also discuss it with your doc who prescribed the meds.

Applies to: promethazine, Suboxone (buprenorphine/naloxone)

MONITOR: Central nervous system- and/or respiratory-depressant effects may be additively or synergistically increased in patients taking multiple drugs that cause these effects, especially in elderly or debilitated patients.

MANAGEMENT: During concomitant use of these drugs, patients should be monitored for potentially excessive or prolonged CNS and respiratory depression. Ambulatory patients should be counseled to avoid hazardous activities requiring mental alertness and motor coordination until they know how these agents affect them, and to notify their physician if they experience excessive or prolonged CNS effects that interfere with their normal activities.

https://www.drugs.com/interactions-check.php?drug_list=1949-0,439-2040