I think the common hiker's "First Aid Kit" is worthless. Here's why I don't typically carry one.

The Boo-Boo Kit

On the subject of FAKs I take issue with the "first aid kits" commonly carried by hikers. I choose not to carry one of these "med kits", and I'm frequently met with comments that I'm irresponsible, moronic, or hiking "stupid light".

I was an intermediate level EMT and Wilderness (WEMT) rescue certified, and in my experience the first aid kits I see in gear lists are effectively 'boo-boo kits', and largely unnecessary and ineffective outside of particular personal needs. Small cuts and scrapes can be managed until I get to a resupply point. A great list of real medical emergencies, not treatable with the hiker boo-boo kit, that you may encounter in the field can be found here.

If you are going to carry a medkit suitable for more than just cuts and scrapes, you'll need to pack kerlix rolls, triangle bandages, plenty of tape, loads of 4x4 gauze pads, a CPR barrier device, materials to seal a sucking chest wound, and more. Plus, you are going to have to know how to use it all! Bandaids are fairly worthless and whether certain OTC meds are 'necessary' is very dependent on your personal needs. In the event of traumatic injury or medical emergency these kits will not be appropriate. There is one good exception to this statement about common FAKs, and that is where people include Aspirin. Asprin's blood-thinning (anti-clotting) properties are often made use of in the event of a heart attack. "Chew two, swallow two" was standard operating procedure in the back of the ambulance. If you aren't 'in the know' already, take a few minutes to read about CPR and heart attacks.

Capability vs. Actual Need

I understand that the "boo-boo kit" the sentiment will often be "at least you can do SOMETHING" especially to help others, but I don't agree that these kits are effective. I think a hiking med kit should be built around YOUR known and persistent needs. On my hikes I rarely go longer than 4 days between resupplies and can pick up whatever I find I'll need going forward in the next town. For example, you know that you have frequent or persistent knee-pain on hikes? Bring OTC or prescribed meds where appropriate. You have terrible  allergies? Bring OTC meds and an epi-pen where appropriate. You have a weak stomach? Bring OTC meds. You get bad blisters? (1) change your footwear, (2) pack a needle and thread in your kit.

If you want to be able to help yourself and others

The FAK is largely a feel-better item unless you tailor it to YOUR specific needs or carry a SERIOUS kit and know how to use it. Some things that I'll tell anyone reading this are (1) don't suffer from the bystander effect of "someone should do something" in an emergency, get in and help, (2) take five minutes to read about controlling severe bleeding, how to apply a tourniquet, and how to improvise a tourniquet. Even just understanding that stopping bleeding usually takes Well-Aimed Direct Pressure can save lives. Finally, (3) You can't pack and prepare for everything. Take a first aid course/ wilderness course if you know your activities will remove you from access to medical care, quick help in the event of an emergency.

Final note

If I am planning to spend time in a very remote wilderness area or am going to be out with a group for more than a week or so without resupply, more than just the normal hiking experience, I will bring a true medical kit capable of addressing common wilderness medical emergencies. Note that this kind of backpacking or camping trip would be well outside of rapid access to medical care, making me truly rely on my own skills and equipment in the event of an emergency.

More importantly, I will ABSOLUTELY carry a PLB or other reliable distress beacon to summon an emergency response where necessary.