It's important that whenever you go into the outdoors, you bring a wilderness first aid kit with you. You can buy ready made kits, but they tend to be expensive and you won't be as familiar its contents as you will be if you build your own. Plus, I have never found a first aid kit I was completely happy with – I always have to add other items. That's why I'd recommend just making your own from the start.
The photo above is from when I took the Wilderness First Responder course with Boreal River. I'd highly recommend taking a wilderness first aid course if you spend time outside.
There are a lot of resources you can use when building your own. Both WildMed (the organization that runs wilderness first aid courses) and REI have great lists for what to keep in your wilderness first aid kit. This list is just to provide a little more background into why you should carry certain items, when you'd need something, and tips for packing everything so it's easy to carry.
Here is a quick (not exhaustive) list of the most common injuries and issues I've experienced on a hiking or paddling trip.
Below is a list of what to include in a wilderness first aid kit. Note: this list is only for personal use on standard camping trips, like paddling and hiking. This list is not sufficient for expeditions, arctic and mountain travel.
What does all the wilderness first aid stuff go into?
You can buy a bag specifically for first aid, or you can keep everything in a big toiletries bag (I'd recommend buying some red duct tape to label it as a first aid kit though). If you're paddling, I'd suggest putting it your first aid kit bag inside a dry sac or pelican case to keep it all safe from water.
Epi-pen. I don't carry this on personal trips unless someone explicitly needs one. Also, some provinces/states require you to have a prescription. However, whenever I do carry one, it is in an extremely easy to find place.
Medical gloves . Preferably use nitrile over latex because of latex allergies. Never do any first aid treatment without wearing gloves!
I keep all of this together in a big ziploc bag labeled "Bleeding and Burns" so it's easy to find.
Antiseptic wipes. These can be BZK or alcohol wipes and are used to disinfect the cuts and scrapes.
Band-aids and bandages of various sizes. Used to prevent further bleeding.
Polysporin (or another anti-bacterial cream). Goes on a clean would, good for cuts, scrapes and burns.
Medical tape. For large cuts or burns, you can cover the entire area with gauze and then secure with medical tape.
Scissors. Good for cutting gauze or bandages, I keep paper towel wrapped around the blades and secured with an elastic band to prevent the scissors from ripping the bag.
Tweezers. Needed for removing splinters.
I also have a separate ziploc bag with polysporin, antiseptic wipes and band-aids that I keep outside the first aid kit (either in my small dry sac if I'm paddling or in an external pocket on my backpack. Small cuts and scrapes are common (especially when guiding camping trips with kids) so I don't want to open up the wilderness first aid kit every single time someone has a minor cut or scrape.
Benadryl (or another anti-histamine). Needed for allergic reactions (and also helpful to bring down swelling and discomfort if you get absolutely eaten by bugs – I wish I didn't know this because of experience).
Advil (or another ibuprofen). Pain relief, good for muscle pain or injury.
Tylenol (or another acetaminophen). Another drug for pain relief, good for headaches.
Gravol. Good for stomach aches, nausea.
Imodium. Helps with diarrhea, something you do not want in the back country!
Chew-able baby aspirin. If you suspect someone is having a heart attack, get emergency help ASAP and give then chew-able baby aspirin (I've never had to do this, but I still keep it with me just in case).
These can go lose at the bottom of the wilderness first aid kit, because they aren't lifesaving or needed for critical care.
Biodegradable camp soap. Just handy to have.
Sunscreen. Leave some in here in case you forget to bring a big bottle.
Aquatabs. These go in your water and purify it, so bring extra in case you run out of clean water or are relying on lakes/rivers for your water.
Energy bars, snacks high in glucose. If someone is low energy or feeling faint, they may have low blood sugar (if I have a diabetic on the trip, I put this at the top).
Extra plastic bags. After I've cleaned a wound or burn, I keep all the garbage in a plastic bag, so it's helpful to carry around a lot of these.
Pen or sharpie marker & small pad of paper or notebook –> may be helpful to write down details of an injury/illness (like the time and amount of medication).
Get a pocket sized first aid manual. Buy this field guide. It has come in handy more times than I can count. It is like your cheat sheet for wilderness first aid. You get it for free if you do a course with WildMed (which I also highly recommend), or you can buy one for $21.
Pro Tip: Get a pill dispenser (you know the kind with little boxes labeled Sun through Sat) and use a sharpie to label the underside of the boxes with names like "Tylenol" and "Advil". Cover the labels with some clear tape so they don't get washed away. Then keep your different pills in there. Write the name of the drug on a little note with the amount (i.e. Tylenol 200 mg) and keep it in the box with the medicines. Pro tip: keep the whole pill box in a ziploc bag and secured with an elastic. This way it won't open up by mistake or let water in.
This isn't everything you could ever possibly need, but it covers a lot of the basics and should be sufficient for a day hike/paddle or weekend camping trip.
I'd also recommend taking a wilderness first aid course. You can never be over prepared in the wilderness! There are a lot of courses, so check out my post on choosing the best wilderness first aid course for you.