Using one of the Reflex Injectors permanently adds an extra bar to the Reflex Meter in the HUD. Each bar means the Reflex ability will last longer, especially towards the end of the game if all Reflex Injector's are found.
There are 14 levels or missions in F.E.A.R. 2: Project Origin, all of which contain a Reflex Injector except for the first. So in total there are 13 Reflex Injectors you can find and use. Additionally, finding all 13 Reflex Injectors will unlock the "All Juiced Up" Achievement on Xbox 360 or "All Juiced Up" Silver Trophy on PS3.
The following walkthrough guide will help you locate the Reflex Injector location in each mission. Once you find the first the others will easy to spot.
Mission 2: Awakening
You'll need a key card to access the room where the Reflex Injector is located. Move past the first locked double doors after the elevator scene and into the next room. The key card is on the dead guy in this room. Get it from him, return to the double doors, open them and you'll find the Reflex Injector sitting on the upper shelf of a shelving unit.
Mission 3: Discovery
Your work moving through the vent will put up in a room with a separate bathroom off to the side. Head into the bathroom and you'll find the Reflex Injector on the floor in the corner.
Mission 4: Withdrawal
Towards the end of the level you'll head down a flight of stairs. At the bottom check the left for a countertop where the Reflex Injector is. If you head into the elevator and go up then you've gone too far.
Mission 5: Replica
The Reflex Injector is through a pair of double doors just outside the Abomination Monitor Room. You'll know the room because you have to turn a valve in it to cut off gas to a flaming pipe blocking access.
Mission 6: Ruin
You'll fight in a building near a downed plane with two stories. Head to the top story and look for a box in the back of the floor that has the Reflex Injector sitting atop it.
Mission 7: Top
After exiting the Power Armor, head left and up a flight of stairs. There's a table in the room at the top that has the Reflex Injector sitting on it.
Mission 8: Elementary
You'll know you're close when in the school building with yellow lockers and the appearance of ghosts. Keep an eye open for a room just after the trophy case on the right. The Reflex Injector is cleverly hidden under a projector in this room.
Mission 9: Nurse's Office
Pay attention for a pair of soldiers getting the wrong end of a flamethrower. After this happens, look for a bathroom on the right where the Reflex Injector is stashed behind a toilet.
Mission 10: Snake Fist
First you will want to stop the lockdown and return power. Once complete, head to the room labeled Chem Lab C-400 and look for a ladder where a mechanical arm is moving about. It's movements will reveal the Reflex Injector to pick up in the middle of the space where the yellow and black caution border is.
Mission 11: Keegan
Head up a staircase and look for another staircase after hanging a right. If that staircase goes down then you'll find the Reflex Injector at the bottom.
Mission 12: Epicenter
You'll encounter a badass with a laser gun just outside the theater. Defeat him and head into his room to find the Reflex Injector on a countertop.
Mission 13: Approach
There is an unmistakable large room in this level with an elevator platform in the middle. You'll need to go up on the platform and ascend/descend a pair of ladders to find the Reflex Injector tucked away in a small nook above the room's entrance.
Mission 14: Climax
You'll encounter "The Tree," and after which traverse a tunnel. You'll spot the Reflex Injector location just after exiting the tunnel.