Q: Can I be a little informal when writing about a previously established fact in the manuscript?

You have provided a template statement rather than the actual statement. So, we may not know for certain whether the said drug is indeed helpful for the said condition. However, we will assume that you know your research well and are therefore accurate in making that statement. :-)

Coming to your query, the statement may not work in a scientific paper for several reasons.

• A scientific paper needs, quite simply, to be scientific. When you write ‘all of us,’ it may not be entirely clear who you are referring to (as included in your query itself): the authors, the readers, the researcher community, or the public at large.
• The statement is indeed a bit informal. This style may be more suited for a press article, but not for an academic article, although some journals may allow a slightly informal style.
• Certain sections of the paper need to be written in particular ways, employing different tenses. It's not clear in which section this statement appears: it could be the Introduction or the Discussion. To know more, you may refer to this resource: Using past and present tenses in research writing

For help with various nuances of scientific writing, you may refer to the resources provided at the end of this response. (For additional help, you may look up the site using the appropriate keywords.)

You would therefore be best in going with the standard way of writing these statements, something like: ‘It is well established that drug D is helpful [beneficial] for XYZ condition.’ What a style like this also does is that it puts the attention on the fact rather than on the writers/readers/researchers/people.

Hope that helps. All the best for your writing!

Some useful resources:

• Is it acceptable to use first person pronouns in scientific writing?
• What kind of sentences are preferred in scientific writing - long or short?
• Learn style and word usage in scientific writing [Course]