Get clear before using a medkit
Use a medkit once you have room to recover and can deal with whatever just damaged you. If you are still in the path of the hazard, move clear first when possible. Then check your health and decide whether you need to heal before entering the next room.
What the preview showed about healing
In an early-build gameplay preview, a player used a medkit after taking damage near exploding boulders. They reported recovering roughly two health pips. That is one player observation from a preview build. Check your own health display after using a kit so you know how much room for error you have left.
Where to look for replacement medkits
The official DERELIKT exploration announcements describe medical supplies as one possible reward for reopening routes with new equipment. When a damaged barrier prevents you from reaching a supply room, remember where it is. If you later find a tool that can clear it, you have a reason to check that route again.
Check your supplies after a difficult encounter. If you are running low on healing and have a known supply detour nearby, it may be worth investigating before you push farther into unfamiliar rooms.
FAQ
How much health does a medkit restore in DERELIKT?
One player reported roughly two health pips in an early build. A fixed healing amount for the release build has not been established by that preview.
When should I use a medkit?
If you have taken substantial damage, get away from the immediate hazard if you can and heal before entering a room where you expect another fight.
Do all reopened routes contain medical supplies?
Medical supplies are one possible find. The developer also mentions ammunition and useful equipment as rewards for exploring reopened routes.