The trick to writing good medicine is starting from what makes sense. There are all kinds of medical mistakes in fiction that get laughed at by those in the know; these tend to go beyond the big ones, such as people walking away from getting CPR, even in the field. I recently watched a movie where a main character had what was called “heart failure” without so much as a cough or a wheeze; they collapsed quietly to the ground and when a monitor was placed on their chest (without exposing the skin, of course), it showed the heartbeat still in a lovely sinus bradycardia, slowly cycling down to zero with the big flashing green numbers growing ever smaller. What did the paramedics do? Chest compressions. No atropine, no external pacers, nothing. Of course the CPR (and a kiss from a lovely woman) brought him right back, at which point he began chasing the bad guys straightaway without so much as a warning from the paramedics that a trip to the nearest cardiology ward might be a bright idea.
Now I’m not saying that one must be a cardiology expert in order to bring a bit of high stakes medical peril into the story, but you do have to have a vague idea of what you are talking about. To continue the cardiac arrest example, only five to ten percent of out-of-hospital cardiac arrests without external defibrillation survive, and even then, survival involves rapid transport to the nearest emergency room with blasts of epinephrine, a search for the cause of the cardiac arrest, and often times a prolonged hospital stay involving deep sedation and therapeutic hypothermia. You could avoid those things for the sake of your story, perhaps, but it would be nice to at least recognize of the seriousness of the event. In the end, of course, it all boils down to doing research and asking for help, if needed and available.
This rule usually applies most when dealing with fiction that takes place in the early twenty-first century with human patients as characters; the rules become more complicated when dealing with fantasy or science fiction settings. George RR Martin’s A Song of Ice and Fire does a great job of dealing with the nasty complications of unsanitized wounds in a medieval setting, and even obliquely references characters having epileptic seizures and inflammatory conditions such as gout, all in terms that make sense given the feudal setting. Many science fiction authors do amazing work in plotting out the various biological facts and medical needs of various alien species or evolutionary offshoots, usually taking off from known species and medicine that we are familiar with in our day-to-day lives and extrapolating from there. But these different styles and genres have that starting points from the well-established medical canon, and then letting the needs of the story take over. One of my favourite writers, who is also a good friend, has characters in her work with plant-based biology, and every time she talks about chloroplasts and carbon dioxide I start to smile. I also forgive any small errors, being in a much more charitable mood after seeing her work earlier in the story.
By now you may be detecting a theme in these posts, and that’s fair. It all comes down to doing your research, knowing (or having a fair idea of) what would happen medically in any given situation, making adjustments for the time and the place that you’re working with, and then deciding how much license your story needs to take with those elements. These really isn’t much point in trying to be pedantic; example above notwithstanding, most people don’t know the elements of advanced cardiac life support and will overlook or forgive most errors. But you don’t have to be exhaustive in order to have at least a partial grasp, and it’s always better in your research to consciously decide to leave details out than not to be aware that they exist in the first place. Even if you’re going the full-out fantasy route and plan to have your healing be with magic rather than moxifloxacin, it’s still not a bad idea to have an idea of what would happen in the medical realm and then figure out a way to have your healer’s spell do the arcane equivalent.
So, do your research, make your adjustments (if the situation calls for it), and don’t overwhelm readers with details – you’re not trying to write a medical textbook, after all! Keep it plausible and grounded in realism, or at least with realism as a starting-off point, and you should avoid the worst of the errors, such as a kiss doing what atropine could not.