This is the full sentence:

> お酒を飲みすぎると、気持ち悪くなる。

なる here means "to reach a certain state." The rest is easy to translate.

The only thing that confuses me is く. What does it mean here and why is it
necessary?
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2018-11-12T08:33:21.797
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2018-11-13T16:55:34.623
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question
grammar particles
Why is it necessary to add く in the following なる sentence?
You have to learn how to conjugate i-adjectives when it comes to expressing
change using なる. I recently answered a similar question, but that was posed in
a different way so let me just get to the bottomline of what you want to know.

The basic construction rule when expressing change with なる is the following.:

> い-adj （－い＋く）＋なる Example たかい >>> たかくなる
>
> な-adj （－な＋に）＋なる Example きれいな >>> きれいになる
>
> nouns （＋に）＋なる Example せんせい >>> せんせいになる

And when it comes to negative:

> い-adj （－い＋く）＋ならない Example たかい >>> たかくならない
>
> な-adj （－な＋に）＋ならない Example きれいな >>> きれいにならない
>
> nouns （＋に）＋ならない Example せんせい >>> せんせいにならない

In your case 悪い is an i-adjective, hence 悪くなる.
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2018-11-12T08:42:17.493
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2018-11-12T08:42:17.493
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answer
Simply put, ～く is one possible conjugated form of i-adjectives, used in
negative forms and when the adjective modifies a verb or another adjective
(examples below).

> くらい → くら **く** ない (negative)
>
> わるい → わる **く** なる (adjective+verb)
>
> おいしい → おいし **く** 食べる (adjective+verb)
>
> たのしい → たのし **く** 過ごす (adjective+verb)
>
> やすい → やす **く** ておいしい (adjective+adj)

etc.
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2018-11-13T13:42:27.573
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2018-11-13T16:55:34.623
2020-06-17T08:18:27.500
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answer