Doctor can be used to combine Ambulance, Cyborg, Hospital, Idea, Penicillin, Scalpel, Statue, and Stethoscope in Little Alchemy 2. Making Doctor is a complicated process, we have figured out two ways to achieve the goal. You need to make Life, and Human first, then combine Doctor with Hospital or Stethoscope. Check out the detailed walkthrough and try it yourself.
Fire + Earth = Lava
Lava + Earth = Volcano
Water + Water = Puddle
Water + Puddle = Pond
Water + Pond = Lake
Lake + Lake = Sea
Earth + Sea = Primordial Soup
Volcano + Primordial Soup = Life
Earth + Water = Mud
Air + Air = Pressure
Earth + Pressure = Stone
Air + Stone = Sand
Sand + Mud = Clay
Clay + Life = Human
Method 1: Hospital + Human
Small + Life = Bacteria
Bacteria + Human = Sickness
Stone + Stone = Wall
Sickness + Wall = Hospital
Hospital + Human = Doctor
Method 2: Human + Stethoscope
Earth + Earth = Land
Earth + Land = Continent
Continent + Continent = Planet
Air + Planet = Atmosphere
Water + Atmosphere = Cloud
Cloud + Cloud = Storm
Storm + Lake = Wave
Air + Wave = Sound
Human + Stone = Tool
Sound + Tool = Stethoscope
Human + Stethoscope = Doctor