It's always /seriously/. I've been wondering why before...

thanks, oreg.

In some dialects, especially US ones, people use adjectives instead of adverbs like this, so it's heard quite often in informal speech. In writing, however, it's wrong.

I'm an American, and I have never heard anyone other than non-native speakers, especially Germans, say "take it serious." It's always "seriously." This is an idiom and has nothing to do with colloquial expressions like "He did good/bad," where an adjectival form is used adverbially. The words that you can use this way in colloquial speech are few, "good" being the most common, and this usage is very common in every English speaking country, not just in American "dialects." If in doubt, always use the correct ending -ly for adverbs.

Neil Diamond sings (Delirious Love, 2006):

Pretty soon we were taking it serious,
Me and and you underneath a mysterious spell,

So, it is used by native speakers.

I agree with Fabian. Some people will undoubtedly argue that the usage is incorrect, but I've certainly heard people say things like "don't take life too serious."

For me, both "serious" and "seriously" are acceptable. The first less formal though, and probably should be avoided in serious writing. (OK for texting and emails though.)

I agree (belatedly) with John. I've never heard anyone say *'take it serious' except pop singers and German speakers. Every time someone does it in this forum, it sounds really bad to me. Nothing to do with speaking vs. writing or informal vs. formal, just plain wrong.