According to WebMD, a gray pill with "K 57" on one side is an oxycodone hydrochloride 20 milligram tablet. DailyMed indicates that it is a narcotic prescribed for moderate to severe pain.
Oxycodone HCl functions like morphine and primarily affects the central nervous and digestive systems. Patients who use oxycodone HCl do not lose consciousness from its effects. Over time, patients sometimes develop a tolerance to the drug, so greater amounts are needed to produce the same amount of pain relief. Common mild to moderate side effects include nausea, constipation, vomiting, headache and itching. DailyMed also lists rare but serious reactions, such as anemia, migraines, increased heartbeat, bone pain, personality disorder and tremors.