We work with a few doctors and you'd be surprised.
Person comes in with medical condition, get prescribed drug, has reaction. Follow up visit, doctor realizes it's a drug interaction.
"Are you on X?"
"Yeah, why?"
"I asked you if you're on medications..."
"Oh I forgot"
It's not always the patient. Mistakes can go both ways.
Several years ago a doctor at the VA prescribed a topical medication for a skin condition. I had a reaction to the medication, so I stopped using it. At the next appointment, the doctor threw a hissy fit when I told him I had stopped. The doctors in the specialty clinics are generally residents, on loan from the nearby medical school, but they are supervised by an attending physician who is on the VA staff. The resident went scurrying out, and came back with the attending so he could (he thought) have THE BIG MAN chew me out for not following HIS instructions. The attending asked why I had stopped using the medication. I told him I stopped because it left my skin dried to the point I had open cracks -- so I stopped. Attending turned to the resident and informed him that stopping a medication when there's an adverse reaction is what a patient is supposed to do
Fast forward two or three years. Back at the dermatology clinic for an appointment with a different resident, for the same condition. He gave me a prescription, which I filled at the hospital's pharmacy and took home. But something about the name of the stuff jogged a couple of random gray cells in the back of my head. I checked the record I keep of my medication history and, sure enough, the new prescription was the same stuff I had the adverse reaction to. I don't know if the new resident missed it, or if the first resident maliciously "overlooked" entering the reaction in my record as revenge for my making him look bad in front of the attending physician.