simple. Put a trigger with the condition-
! canstand player && (alive player)
on activation-
medkit = player addaction ["Use Medkit", "medkit.sqs"]; hint "Medkit available"
Make a new text file and call it "medkit.sqs"
Contents of medkit.sqs-
player removeaction medkit
player setdammage 0
~1
hint "Health Restored"
exit