There are a lot of monsters in "No More Room in Hell 2". If you don't pay attention, you will be defeated. If you want to heal your teammates, you can get close to the character and press the E button to give them medical treatment. Aid, or if you have enough supplies and your teammates can benefit from them.
How to heal teammates in Hell is Full 2
Hell is Full 2 is more than just a single-player experience. Realistically, you need to keep as many of your friends and other responders alive as possible if you want to have any chance of survival. You can contribute to your team's success by treating the injuries of your teammates.
If you have a health kit or bandages on hand and your teammates don't, or they request help via the microphone or in-game text chat, you can give them medical aid by getting close to your character and pressing the E key. Alternatively, if you have enough supplies that your teammates could benefit from, you can press the Tab key to open the backpack and right-click the item to drop it so they can pick it up.
