they are used interchangeably in the UK by lay people, and also often by doctors or people in the medical profession. However there is a very subtle difference that sometimes means one word is preferable or the correct word to use in certain situations. Sick British English = to vomit. Sick British English dictionary = affected by physical or mental illness. Sick of British English = to be bored or annoyed with someone or something. Ill = suffering from illness or a disease ill =of poor quality ill= badly wrongly or imperfectly "he was taken ill with food poising" = he felt unwell "he was taken sick with food poisoning" = the same but taken ill is more natural and instinctive for older speakers like me, but "taken sick" seems to be creeping into the language. British English: we suffer from mental illness (not sickness) we take sick leave from work (not ill leave) we fell sick or ill when we have the flu. we say a person is very ill or very sick. (sick has more of a meaning of felling unwell) (ill is used for serious diseases more often) a patient in hospital with severe cancer: a doctor might say very ill in a professional situation to indicate the seriousness of it. Patient with an upset stomach: ill would be more formal but a doctor might say sick to indicate just feeling unwell for a while. Apparently America often uses sick where British use ill and vice versa. Dictionary definition if you are sick you are ill. The two words are increasingly becoming always interchangeable in professional and lay persons terminology. And America and Britain apparently apply the words a little differently but not that it would make communicating confusing, it would be just like adjusting to local area dialects. My edit = I Changed this "The two words are becoming increasingly always interchangeable" to what is above i Think the above is better. "The two words are increasingly becoming always interchangeable"

In New Zealand (and I think Australia) we don't usually use "Ill'.

"Sick" is common informal English. "Ill is more formal and is not used often in spoken American English (not sure about the British). In most cases, both words can be used interchangeably. I was sick yesterday. He called in sick. He's taking sick leave. I feel ill. My grandmother was taken ill with pneumonia.

In the United States my friends and I would probably use the word ill to indicate a chronic or long term sickness, but this is still more formal and a bit out of date. I would probably say "they have not been well" and people would understand I meant "ill" or sick for a long time. Sick can have many meanings based on context or usage. He is sick means a short term illness. She has been sick means she has been ill for a while or a long time. I feel sick means a recent or temporary state of not feeling well ( maybe I ate something that disagreed with my stomach!). I am about to be sick or I was sick, could mean I am going to vomit. There is even the slang use of "sick," as in, "wow, that's sick!"which means that's great. So many meanings for one little word!