It isn't a medical condition. Narcissistic personality disorder is rare condition, diagnosable only by a physician.
Its fine to describe someone as narcissistic, if you are just saying that in your opinion they are attention seeking and ego centric. However the way the word is used on mn sometimes, it is as if some people think they automatically win an argument by labelling their mother/ partner/ sister/ friend narcissistic,
You are not able to state that a person actually has something pathologically wrong with them, rather than just saying "I don't like this person very much" , in many cases a fr more objective description of the situation.