If they were immediately addictive, everyone that got a morphine shot in the hospital would need to seek treatment. It isn't a "one use and you are addicted" or even "occasional (recreational) use will get you addicted". A portion of folks will seek it out more often, but most folks will just seek it out occasionally.

With opiates, the bigger danger is daily or consistent use over a period of time. The period depending on how often you take it and how you are using it and the strength of your dose.