So you made a mistake...
Always remember that you will save, help and comfort vastly more people than you will ever harm.
So you made a mistake. I know you’re busy beating yourself up about it. I know that after years and years of training in life-saving medicine you’ve also trained yourself to accept the blame for all sorts of things beyond your control.
You’re asking, “How could I have missed that,” or “Why didn’t I think about that?” You’re wondering why you didn’t give a different drug,