Difficulty Level and Health and Damage (DXHR)

This article borrows from Absolute Steve's great DX3 FAQ

Health and Damage
Jensen's health is fixed at 100 points at Normal Difficulty. This can be increased up to 200 using Painkillers (25 points), Hypostims (50 points), and drinks. Anything over 100 is temporary and lost if you are injured. If Jensen's health is below 100, Hypostims and Painkillers first bring it to 100, then add their 25 or 50 points. Drinks do not bring health to 100 first:

At Normal Difficulty, his health regenerates 4 points per second, after a delay (without further injury) of 6.5 seconds. Thus, it would take him 31 seconds to recover from 1 hitpoint to 100 HPs (6.5 + 99/4).

Enemies also regenerate health. At Normal difficulty, they recover 1 HP/s after a delay of 10 s. So it would take them 109 seconds (almost 2 minutes) to recover from 1 HP to 100.

Difficulty Level
Difficulty level affects you and enemies as follows:

"Regen 99" is the time it takes to go from 1 Hitpoint to 100 HPs.

The game will still show you as having 100 base health at each difficulty level, with a max of 200, but the underlying number of hitpoints represented by that number have changed. Think of your in-game health bar as "percent health" relative to the hitpoints shown above.

The various enemies have differing numbers of hitpoints, depending on their size (e.g. unarmored civilian versus ogre), difficulty level (as shown above), and whether it is a head shot or body shot.

After you and enemies, there are only a few other objects that can be destroyed:

Difficulty Level does not seem to affect the damage from weapons; it only changes how much health (hitpoints) you or enemies have. Since you have less health and enemies have more health at higher difficulty, you still die easier and they are still harder to kill, despite the "fixed" weapon damage.

For example, the machine pistol (MP) with no damage upgrade (D+0) does 1.75 damage with each round, and the body of a Boxguard has 380 HPs at Normal difficulty:

Conversely, a Door, which has 150 HPs at all difficulty levels, always needs 86 MP D+0 rounds to kill (150/1.75).