How soon does Metformin take to work?

I don't believe there are any absolutes on this but I would tend to wait another week or so. Don't expect too much from just the one tablet. Wait until you add the second one. I assume you are also following a suitable diet as well. Diet, exercise and tablets all need to be taken together to have a measurable result. Good luck.

Purely anecdotal, but I don't think I saw any impact on my BG levels for about a week after I started taking Metformin. I remember being chuffed when I got my first 5s...

Having said that, Metformin isn't going to put much of a dent in a "15 mmol/l", you might need to help it along a bit with some carb counting.

Good luck.

I went low carb, and my levels fell when I worked out what I could eat and couldn't, along with the tablets. About a week or two with trial and error.

Ha ha! I must be the exception to the rule then, as I so often am.

My Metformin was gradually increased over a period of 3 months. I ended up on 2000mg per day. It took 2 and a half months from starting it, before my BS came down and became stable.

Helena.