Efficacy differences in metformin brands

Type 1 here, MDI using Novolog via InPen and Tresiba. Medtronic CGM. Also take metformin to help with insulin sensitivity (500mg twice a day).
In mid August, there was a recall on a particular brand of Metformin due to trace amounts of MDMA. I must’ve been taking the brand because my next prescription said ‘new brand.’ Didn’t think anything of it until the last couple weeks where my insulin sensitivity has been plummeting. Really, I saw a slow and slight decrease over the past month or so since the change in Metformin brand, with a large change the last 2 weeks. We’re talking like insulin needs of what I was taking before metformin (at least 25+%). I haven’t changed my diet, my exercise, macro ratios- nothing I can think of to affect insulin sensitivity other than this change in metformin brand.
Has anyone had this happen to them where one brand of metformin performs differently for them compared to another? I know they’re supposed to be the same, but we all know how that goes with insulins too, so I’m curious if this could be the same scenario.
I have a call into my doc to see what to do (other than obviously raising insulin ratios/dosages), but she will take a bit to respond, and I’m curious if anyone else has seen this.

Or try increasing metformin dosage?

I also use metformin for same reasons, but was not impacted by the metformin recall.

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@MM1 thank you, yes, that is also an obvious step I should have mentioned. Currently not planning on this route, but I’m well aware it’s an option.
I’m less concerned about what to do rather than why it’s happening. There’s literally nothing to explain why I have this relatively sudden (in regards to how long metformin starts to fully work) and extreme change in sensitivity, other than (potentially) the change in Rx brand.

Try Glucophage. It’s more expensive than generic but probably more consistent. Bernstein will only prescribe that.

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Extended release meds tend to be the least consistent across generics, because what makes them ER is the fillers etc that can be different from brand to brand. Same thing is true for psychiatric meds that are ER–it’s why for ER meds, some people prefer brand name, and it’s one of the times that is likely a reasonable preference IMO. Your dr might have to write a letter for your insurance to request coverage, but if you can document the change in control, could work.