Pholcodine is the active ingredient in many over the counter cough preparations.  Pholcodine is an opioid medication that is used to supress dry and tickly coughs. 

The European Medicines Agency (EMA) conducted a review and found a link to the consumption of pholcodine and the increased risk of anaphylactic reaction to general anaesthetic agents.  

More information is available from teh TGA website here

https://www.tga.gov.au/news/safety-alerts/pholcodine