Disclaimer: I am not a doctor. I do not intend to act as a doctor. This is not medical advice. This is not even a first aid class. This is just my account of what I brought in my first aid kit on the Pacific Crest Trail. It worked out for me, but the same combination of items may not work out for you.
A first aid kit should be designed for its stated purpose: providing the first aid you use in an injury or illness before full medical treatment is available. So the key with first aid kits on trail is that they should have the essentials while not acting as a mini-hospital. You have to carry everything after all.
You'll note just by glancing at the contents of my first aid kit that a lot of the items were in fact more for comfort than for emergency medical situations. While you are going out into the wilderness, you are usually not very far from civilization, especially if you carry a GPS device (like the SPOT or Delorme InReach) that allows you to push an SOS button to request a medical helicopter. I did not carry one, but I often hiked with others who did.
When I put together my kit, I was fairly minimalistic (and optimistic). Some factors worked in my favor: I only got three blisters on trail, the only bone (I am 99% sure) I broke was my tailbone for which there's really no treatment according to my rigorous on-trail WebMD research, and I am young which is quite beneficial for a long distance trek. You'll also notice that my first aid kit was also my repair kit and hygiene kit. No need to separate little, loose items.
What Was in My First Aid Kit
A bag full of pills
Ah, my bag of loose pills. My little treasure trove. The three main pills I carried were Tylenol, ibuprofen, and Benadryl. I carried the Tylenol (AKA "tylie") for when the pain every day became intense in NorCal with our long miles. I carried ibuprofen for the same reason, but used it less frequently because it wasn't quite as effective as Tylenol. I carried Benadryl for any allergic reactions that might happen on trail. The little pink pills became very useful when I discovered an allergy to Juniper trees on the second day.
Campho-Phenique is the most effective antidote to mosquito bites, and I will fight anyone who challenges me on this. I carried this little miracle worker the entire time. I should be hired as their spokesperson because I am radically supportive of this product. It comes in both a liquid form (as shown in the bottle) as well as a cream form. I started with the cream form and when that ran out, I picked up a bottle. They don't sell the cream form (which is lighter than the liquid form) on trail, as far as I could tell. The best way to use Campho is to apply it directly to the skin as soon as you notice you've been bitten by a mosquito. If applied early on, you will hardly notice it. I am very sensitive to mosquito bites and often would have large, swollen areas of itchiness. I did not have this effect if I applied Campho, even a few hours after the immediate bite. I 100% recommend this product if you are sensitive to mosquito bites.
I picked up a bottle of probiotics when I was having digestion problems in the Sierra section. I would take one every day. I'm not sure if it helped all that much, but I liked the idea of taking in some good bacteria considering all the garbage food I was putting into my body day after day. I would also recommend eating yogurt when you are in town.
I carried Neosporin antibiotic ointment in the event that I got an open wound or was doing blister treatment. Scratches and cuts on your legs were common in the Sierra section when we had to climb over fallen trees. When I used to pop and draw out the fluid in blisters, I cleaned the area with alcohol wipes and Neosporin.
If I needed an extra layer of anti-itch treatment in addition to my Campho-Phenique, I would liberally apply hydrocortisone cream. As you can see, I didn't use it often because Campho is life.
Alcohol wipes serve two purposes: first aid and equipment repair. When I got a particularly rough cut climbing over fallen trees, I would clean the wound with alcohol wipes. Honestly though, my legs were so cut up that I usually just left them to scab over. I only used a couple alcohol wipes for larger gashes. Alcohol wipes are also used to clean the area around broken gear, so that the adhesive of Tenacious Tape (see below) or other repair tape would adhere properly.
This is the repair kit that comes with a Therm-a-rest inflatable pad. In the event that there is a hole in your pad, you can patch over it with the contents of this kit. I never used it, but I figured it was worth carrying.
Sewing up broken gear is a common occurrence, especially several hundred miles in when your gear may be worse for wear. I acquired these needles and thread from a zipper repair kit that ended up being useless in the task of repairing my tent zipper. I tossed the zipper replacements and kept the needles and thread. They were useful when I had to sew up my gaiters a few times.
I would not hike without a back-up water treatment tool. Water-borne illnesses are common on trail, namely Giardia. In the event that I lost or broke my main water treatment, my Sawyer Squeeze, I would be able to use these iodine tablets to treat my water. Pro tip for anyone who is getting really nuts about weight: Do not take iodine tablets out of the glass bottle. I thought I could cut weight by putting them into a plastic bag and it ended up causing my desk to become discolored.
Half hygiene / half repair. I flossed religiously on trail. Some might see that as super weird, but I kept up my dental hygiene because it seemed to be the only hygiene I could keep up on trail. In fact, I think my teeth had never been cleaner. If you are out of thread, you can also use floss to repair gear. I hear it's actually stronger than thread.
Half of a toothbrush (not pictured)
First off, sawing off half your toothbrush is a very fun pre-hike chore. You can brush your teeth with half a toothbrush just fine. If that's not your jam, others often carried a child-sized toothbrush.
Sunscreen (not pictured)
Put on your sunscreen. Save your skin. You are probably more likely to develop skin cancer than fall off a mountain side. I put on sunscreen every day, often twice a day. I carried a 3 oz bottle that I refilled.
Lip balm (not pictured)
Fun fact: Your lips can get sunburned. I recommend getting a lip balm with SPF protection so you don't have trouble falling asleep because your lips hurt so much. I used this product and was very happy with it.
Tenacious Tape (not pictured)
This tape is the most common repair tape on trail. It is most often used to patch up holes. I used Tenacious Tape to repair my tent when it ripped at the base and zipper area, as well as when I ripped my rain pants when glissading.
What Should Have Been in My First Aid Kit (kind of...)
Honestly, I was fine. I was apparently not very injury-prone (and very, very, very grateful for that).
Here is a list of other items that were commonly found in others' first aid kits:
Leukotape is an extremely sticky tape that works well both as a first aid tape and repair tape. Its superpower is that it can even stick to sweaty, sandy feet for blister protection. Regular athletic tape isn't up to par. I also saw it being used by one of my trail family members to Macgyver his microspikes back together right before we ascended our most technically challenging and dangerous Sierra pass, Mather Pass.
Way more alcohol wipes
I saw people carry way more alcohol wipes than I did. My hiking partner experienced awful blisters, especially at the beginning, so she wasn't constantly cleaning and treating her blisters. You probably should pack more than three at a time.
This was actually something my hiking partner carried, and she used it on our fourth day. Ever slammed the back of your head into a Joshua tree? You bleed profusely! I honestly don't know what we would have done without this seemingly overzealous first aid kit item. It worked like a charm. I don't know the exact product she had, but here is an example.
I actually carried some of these for a while at the beginning, and then they crushed into a fine powder amongst the other loose pills. In the event you get Giardia on trail, I'd imagine these would be helpful.