Metadol 100 for relief of severe pain. Metamorphone is a man-made opioid that work like morphine and is also used to treat moderate to severe pain. Metadol 100mg Tablet is used in the treatment of pain due to muscle spasms. Some of the common side effects on of this medicine includes nausea, vomiting, stomach pain, heartburn, diarrhea, sleepiness, and loss of appetite.