It's called esophageal dysphagia and it's cause is similar to the reason you get constipated on opioids. The opioids is binding to the mu opioid receptors in the esophagus (or bowels) and causing dismotility. Regarding the swallowing, it's a serious issue that does not remedy on it's own and does not improve over time or with tolerance to the opioids. Eventually, food will get completely stuck (particularly chicken, steak, bread, etc.) not allowing even liquids to pass and require an endoscopy to remove the food bollus. People on longer acting opioids like methadone and Buprenorphine ( Subutex/Suboxone) are more likely to experience this problem as the dose is higher and while you don't get the euphoric intoxicating effects anymore you do get all of the other effects like CNS depression. The fact that they are maintenance drugs so you can't just stop them is all the more reason to discuss a solution to this problem with your primary care provider. I can tell you from personal experience that this problem only gets worse and the only solutions are to stop taking any opioid, at which time the problem completely disappears, or discuss medications that may alleviate the issue with your doctor.