weaken - lessen the strength of; "The fever weakened his body"

water down - make less strong or intense; "water down the mixture"

stretch, extend - increase in quantity or bulk by adding a cheaper substance; "stretch the soup by adding some more cream"; "extend the casserole with a little rice"

corrupt, spoil - alter from the original

water down - thin by adding water to; "They watered down the moonshine"

doctor, doctor up, sophisticate - alter and make impure, as with the intention to deceive; "Sophisticate rose water with geraniol"