at first/in the beginning

[joham]

In the beginning/At first, the medicine will make you fell a little tired and drowsy, but after a couple of days you'll begin to notice an improvement in your stomach.

Is 'at first' and 'in the beginning' interchangable in any context?

Thank you in advance.

[charlieten]

I think so. I think they have the same meaning.

[Route21]

Hi charlieten

Yes, they may have the same meaning, but "In the beginning" has, to me personally, some overtones related to the creation of the earth.

"At first"/"initially" would be fine and would, I believe, be better than "at the start". Of the two, "initially" works best for me.

Hope this helps
R21