• Be a team player
  Don’t keep stims just for yourself. R6 Siege is a team-based game, so keeping your teammates alive and in good health condition is vital to your success.

• Body parts don’t matter
  Heal applied from stim pistol is the same regardless of body part hit by it.

• Make Stim shots count
  Bear in mind that you have only 3 stim shots, so make sure you get the most out of them.
  If you want to heal your teammate, give them heads-up so that they stand still – “Bandit, don’t move” will do!

• Use stim to pick yourself up
  Doc is the only operator on defense who can pick himself up from being injured. You can do so by using 1 ammo from your stim pistol. You cannot revive yourself when you run out of stims!

• Overheal yourself in crucial moments
  Overheal yourself whenever you anticipate a firefight. This might come especially handy when spawn peeking or when the timer is running low, and attackers push is inevitable.
  Those extra few points of health may be a difference between a win or lose.

If a teammate has a tiny amount of health (say 5hp), then down teammate first and use Stim to revive them.
Your teammate will benefit from having 75HP rather than 45HP, with the same amount of Stim used.
Though keep in mind that every operator can be downed just once per round!