to dishonestly change something in order to gain an advantage:
He had doctored his passport to pass her off as his daughter.
There are concerns that some players have been doctoring the ball.
to add something harmful to food or drink:
Paul suspected that his drink had been doctored.
to remove part of the sex organs of an animal to prevent it from having babies [= neuter]:
You should have your cat doctored.
to give someone medical treatment, especially when you are not a doctor:
Bill doctored the horses with a strong-smelling ointment.