Final answer:
A drug that mimics the painkilling properties of morphine is called an analgesic. Opioids are a subset of analgesics that strongly alleviate pain and can lead to euphoria due to their action on opioid receptors.
Step-by-step explanation:
A natural or synthetic drug whose actions are similar to the actions of morphine is correctly referred to as an analgesic. These substances are designed to reduce pain without causing the loss of consciousness. A subset of analgesics, called opioids, includes drugs like heroin, morphine, methadone, and codeine. Opioids are known for their painkilling effects and ability to produce euphoria due to their interaction with the body's endogenous opioid receptors. It is important to note that while drugs such as acetaminophen are also considered analgesics, they may possess different activity levels in terms of anti-inflammation and antipyresis. Nonetheless, the primary comparison here is with the painkilling property that is potent in opioids and mimics morphine, making 'analgesic' the correct answer.