To takе slееpіng tablеts іs harmful = correct but very unnatural Taking slееpіng tablеts іs harmful = natural English Both are correct, in theory, but the first is very outdated. You come across this use of to + infinitive at the beginning of a sentence in old-fashioned expressions such as 'To err is human, to forgive divine', but this construction is very rarely used in modern English. These days, we nearly always use the gerund when the verb is the subject that begins the sentence. Note that we do use the infinitive in sentences such as 'It is harmful to take sleeping tablets'. It's fine to put 'to take xx' at the end of the sentence, where the sentence begins with the dummy subject 'It'.

Actually they are both good, but the participle form is more usual in such kind of sentences. Taking sleeping pills is harmful for your health. - "take" is not a verb here, but more like a noun, describing an action. To take sleeping pills is harmful. - the focus is on the action or purpose.

Again. Here I see no difference. I believe people would understand it the same.