It's a strong painkiller. If you are in significant pain, it will relieve or reduce that pain. It may also make you drowsy, so it's not wise to drive or do other activities that require alertness. Most people find that opiates make them constipated, so if your doctor prescribes them, she may also prescribe or recommend a laxative.
For some people who have brains wired that way, opiates (prescription or otherwise) can cause a feeling of calm or euphoria -- which can lead to mis-use. A few people can become addicted to opiates within a few days of use. Most people (even those with brains wired that way) will need longer. (And for everyone, long use leads to physical dependence and tolerance.) Which is why when doctors prescribe opiates now, they usually prescribe for a few days to week or so -- depending on the source and type of pain.
(I personally don't like the way opiates make me feel. So when I'm prescribed them, I take as few as possible, for as few days as possible.)
And if you are not in pain and not prescribed it -- there is no reason to be taking it, and no legal way to access it, so the question is meaningless.