Golden Rule: Safety first. You have several options available to you for your choice of First Aid Kit depending on how much room you have and the kind of activities you will be doing.
Advanced First Aid Kit
If you want something more heavy duty and are planning on being off the beaten track you will want something like the Solo Traveller First Aid Kit by Lifesystems.
This First Aid Kit contains 49 items and is designed with the adventurous, independent traveller and backpacker. This kit combines all the basics for treating minor, and some more serious, injuries with a full set of sterile medical equipment, including needles and syringes. This kit weights 522g.
Alternatively the World Traveller First Aid Kit by Lifesystems is also suitable for the adventurous traveller on extended trips to exotic or remote locations. This First Aid Kit contains 64 items and weighs 685g. Both of these kits are heavy duty and would only be required in inaccessible or remote areas of travel.
Mid-Range First Aid Kit
If you’ve got a bit of space in your backpack and want a decent sized First Aid Kit then the midrange option from Lifesystems would be the Explorer First Aid Kit. This kit is great for the outdoors, contains 36 items and weighs 395g.
Basic First Aid Kit
A very simple and compact First Aid Kit is the Trekker by Lifesystems which I personally use due to its size and weight of 249g. This First Aid Kit contains; Primary care leaflet, small low-adherent dressing, woven bandage & vinyl gloves, small crepe bandage, fabric plasters, micropore tape & gauze swabs, antiseptic cream, antiseptic wipes, scissors & tweezers, safety pins.
Gap Year Escape recommends the Lifesystems Trekker First Aid Kit