Once you take damage in DEEEER Simulator, you can’t heal unless you make it to the next level, die, or reset your progress by playing Cowthello or quitting the game. There are no health packs or healing items in the game.
The problem with healing through any of those methods is that you lose all of your weapons and Deerstruction Level for that stage in the process (however, you won’t lose access to Metal Deer in Level 1 for instance). There’s no other way of regenerating health.
But what about getting more health in DEEEER Simulator? It does seem that you can get more hearts for more health by dying over and over. However, it’s not really worth the effort. To test, I died 10 times to mobs in the first level and 10 times to Doggo and didn’t get a single extra heart. I did the same to the mobs in the future with the same result.
The only time I received an extra heart for more health was when fighting Future Self and died several times (about five, I think). The extra heart certainly helped in the fight, but getting there on three without any difficulty modifiers is possible.
Perhaps future versions of DEEEER Simulator will add ways to heal within levels that don’t reset progress, and perhaps getting more health won’t require an unnecessary grind. But we’ll just have to wait and see.