Opioid addiction will have different effects on different people. How well they function in social situations, in relationships, in their work/professional life, etc will depend a lot on how their personality was beforehand and the unique situations to their life. Generally though it leads to very inward thinking, which to other people seems like plain selfishness and being self-absorbed. Often times the slightest inconveniences, or everyday frustrations, cause outbursts of anger/hostility/bitterness. At times emotionally numb/blunted, other times raw like an emotional open sore. Unless the person is on a maintenance level of use (having enough money and the connections to get their opioid of choice regularly for a long period of time), the High - Low - High - Low dynamic leads to what seem like mood swings or complete changes in personality from one hour to the next, or day to the next, depending on how much of their opioid of choice is consumed and how often (or in the reverse, how little is being consumed not often enough) can seemingly change their personality. I think opioid addiction is your body and mind seeking stasis- a constant level of opiation all the time, which is very difficult to accomplish given the prohibition of opioids, their high black market value/cost, precarity of sources, etc. This leads to a lack of stasis which goes against what the mind and body crave, leading to the kinds of emotional outbursts, short fuse, letting personal relationships, obligations, etc get left behind, and so on.
It's a torment to experience, and I can imagine a confusing, scary, frustrating nightmare to watch and be around when it's someone you care about or love.