"These two medications might interact with each other? Woopsy daisy, we forgot to tell you that!"
That can be very serious. In my situation, the antibiotic I was prescribed for the urinary tract issues has a known interaction with St. John's Wort (which I have been taking for depression). I already knew this interaction, and I specifically told the staff that this supplement has many interactions during my appointment in anticipation that they would let me know to stop using it while I'm taking the antibiotics. They did not tell me to stop using it. I did this of my own accord because of my knowledge.
Had I continued using both the antibiotic and St. John's Wort, the efficacy of the antibiotic would be reduced, thus rendering the treatment wholly ineffective, as the suspected bacteria would not be completely eradicated. This would lead to the survival of stronger bacteria and worse suspected infections down the line. That would be a pretty bad long-term outcome (and it's also the reason that it's important for people to complete their complete course of antibiotics even if they feel better!).
This is why it's important to know common drug interactions. Urologists typically don't prescribe a great deal of medications (a few select antibiotics, prostate / urethral medications, possibly hormonal treatments, and some supplements), so it's vital for them to be well aware of common or significant interactions.
Primary care physicians, who typically prescribe a much broader range of medications, would probably not be as well-versed in the details of a lot of medications simply due to the vast amount of information required. Hence, specialists and subspecialists should know their common medications like the backs of their hands.
