なくちゃ (a contraction of なくては) and なきゃ (a contraction of なければ) are two of the most frequently used colloquial forms for expressing that something 'has to' or 'must' be done.

Just like their full-length counterparts なくてはいけない (must) and なくてはならない (must), these shortened forms can be paired with いけない, ならない, or だめ. In casual conversation, however, these endings are frequently dropped with no change in the fundamental meaning of obligation.

Grammar like this is characteristic of spoken/colloquial language. The casual feel of these variations is very similar to using a word like 'gotta' in English instead of 'have to'.

• 明日(あした)は早(はや)く起(お)きなきゃだめだから、もう寝(ね)るね。
  I have to get up early tomorrow, so I'm going to bed now.
• 薬(くすり)を飲(の)まなくちゃいけない。
  I've got to take my medicine.
• あっ、牛乳(ぎゅうにゅう)を買(か)わなきゃ。
  Oh, I gotta buy milk.
• そろそろ部屋(へや)を片付(かたづ)けなくちゃ。
  I should probably clean my room soon.

In the sentences above, you can see how the different patterns are used in a natural way. No single form is inherently better than another; the choice is often a matter of a speaker's personal habit.

As is true for many grammatical patterns, the shortest forms (the ones without いけない, etc.) are the most informal and are used most frequently in everyday situations.

A few other variations of these common abbreviations exist, influenced by region or a speaker's individual style. One such form is なけりゃ, a very informal version of なければ.

• 約束(やくそく)したんだから、行(い)かなけりゃだめだろ。
  You made a promise, so you have to go!

This example demonstrates that some of these less common forms are often linked to a particular gender (in a general sense). なけりゃ, for example, is primarily used by male speakers and can have a rougher nuance than なきゃ or なくちゃ, which can be used by anyone.