Most of them can be used once while others can be used twice.
To restore your health walk up to any of these , then press your use key (E by default), your health will instantly recover. If it is a one use cache, all medical supplies will disappear. If it is a two use cache you will have to aim at one of the halves to use it then the other one. One use caches can restore 50 health points and two use caches can be restore 100 health points.