Metformin Microdosing Protocols: What the Evidence Actually Shows

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Is there such a thing as a metformin microdosing protocol?

The labels do not describe one. Immediate-release metformin starts at 500 mg twice a day or 850 mg once a day with meals, rises by 500 mg a week or 850 mg every two weeks, and tops out at 2,550 mg a day; doses above 2,000 mg may be better tolerated split three times a day [2]. Extended-release starts at 500 mg once daily with the evening meal and increases by 500 mg a week to a maximum of 2,000 mg once daily [3].

What gets called microdosing is usually one of two things: a daily dose below 500 mg, or the first few weeks of an ordinary titration. The label describes only the second, as a step on the way up.

What the lowest-dose trial showed

The smallest dose tested in the placebo-controlled dose-response trial was 500 mg a day. That study was a 14-week, double-blind trial in 451 adults with type 2 diabetes and fasting plasma glucose of at least 180 mg/dL, randomized to placebo or metformin 500, 1,000, 1,500, 2,000 or 2,500 mg a day [1].

Fasting glucose fell by 19 to 84 mg/dL more than placebo across the 500 to 2,000 mg doses, and HbA1c by 0.6% to 2.0%; all of those differences were significant except the 500 mg comparison for fasting glucose at endpoint (P=0.054) [1]. The investigators concluded that benefit appeared with as little as 500 mg and was greatest at the top of the recommended range [1].

Stomach problems in the trials

In a US trial of immediate-release metformin up to 2,550 mg a day (141 patients vs 145 on placebo), diarrhea occurred in 53% vs 12%, nausea or vomiting in 26% vs 8%, and diarrhea led to discontinuation in 6% [2]. In placebo-controlled extended-release trials (781 patients vs 195), diarrhea occurred in 10% vs 3% and nausea or vomiting in 7% vs 2%, with diarrhea leading to discontinuation in 0.6% [3].

In the dose-response trial, treatment-related adverse events, mostly digestive, occurred in 28% of metformin patients versus 15% of placebo patients [1]. A 24-week double-blind study compared extended-release metformin at 1,500 mg once daily, 1,500 mg twice daily and 2,000 mg once daily with immediate-release 1,500 mg twice daily; overall adverse-event rates were similar, but fewer patients stopped extended-release because of nausea during the initial dosing period [4].

What the diabetes prevention trials used

The Diabetes Prevention Program randomized 3,234 adults with elevated fasting and post-load glucose to placebo, metformin 850 mg twice daily, or an intensive lifestyle program [5]. Over a mean 2.8 years, diabetes incidence was 7.8 per 100 person-years with metformin versus 11.0 with placebo, a 31% reduction (95% CI 17% to 43%); lifestyle reduced incidence by 58% and was significantly more effective than metformin [5]. At 15 years the metformin reduction was 18% (hazard ratio 0.82) versus 27% for lifestyle, with cumulative diabetes incidence of 56% with metformin, 62% with placebo and 55% with lifestyle, and no overall difference in microvascular disease [6].

During the double-blind DPP, body weight fell 2.06% with metformin versus 0.02% with placebo, and waist circumference by 2.13 cm versus 0.79 cm [7]. Among 1,066 participants who lost at least 5% of body weight in year one, the weight loss maintained between years 6 and 15 was 6.2% with metformin, 3.7% with lifestyle and 2.8% with placebo, a post hoc analysis of non-randomized subsets [8].

What the off-label trials used

Metformin is not labeled for PCOS or for muscle-related uses; those are off-label [2]. In the MASTERS trial, 46 adults aged 65 and older took metformin 1,700 mg a day and 48 took placebo during 14 weeks of supervised resistance training. Placebo gained more lean body mass (P=0.003) and thigh muscle mass (P<0.001), and the trend toward blunted strength gains with metformin did not reach significance [11].

In PCOS, a 2012 Cochrane review of 44 trials (3,992 women), 38 of them of metformin (3,495 women), found no evidence that metformin improved live birth rates, alone or with clomiphene, but clinical pregnancy rates were higher with metformin than placebo (pooled OR 2.31, 95% CI 1.52 to 3.51; 8 trials, 707 women) [12].

Where these trials report a dose, it was 500 mg a day or above: 850 mg twice daily in the Diabetes Prevention Program [5], 500 to 2,500 mg a day in the dose-response trial [1], and 1,700 mg a day in MASTERS [11]. They do not establish a metformin microdosing protocol below 500 mg a day.

Kidney function, B12 and lactic acidosis

The label ties metformin use closely to kidney function. It says to check eGFR before starting and at least yearly; metformin is contraindicated below 30 mL/min/1.73 m2, starting is not recommended at 30 to 45, and if eGFR falls below 45 the benefit and risk of continuing are reassessed, with discontinuation below 30 [2]. The label says to stop metformin at the time of, or before, iodinated contrast imaging in patients with eGFR 30 to 60, a history of hepatic impairment, alcoholism or heart failure, or when contrast is given intra-arterially, to re-evaluate eGFR 48 hours later, and to restart only if renal function is stable [2].

The boxed warning is lactic acidosis. Reported cases were characterized by blood lactate above 5 mmol/L, anion gap acidosis, a raised lactate-to-pyruvate ratio and metformin plasma levels generally above 5 mcg/mL; risk factors named in the warning include renal impairment, carbonic anhydrase inhibitors such as topiramate, age 65 or older, contrast studies, surgery, hypoxic states such as acute heart failure, excessive alcohol intake and hepatic impairment [3]. The label describes the onset as often subtle, with malaise, muscle pain, respiratory distress, drowsiness or abdominal pain, and directs that metformin be stopped and supportive care given in hospital, with prompt hemodialysis [3]. Alcohol potentiates metformin's effect on lactate metabolism, and the label warns against excessive intake [2].

Vitamin B12. In 29-week trials, subnormal B12 levels developed in about 7% of patients, and the label says hematologic parameters should be measured annually and B12 every 2 to 3 years; it attributes the fall possibly to interference with B12 absorption from the B12-intrinsic factor complex [2]. The Diabetes Prevention Program B12 analysis compared metformin 850 mg twice daily with placebo for 3.2 years, then open-label metformin for 9 more years in the metformin group. Low B12 (203 pg/mL or less) was found in 4.3% of metformin users versus 2.3% of placebo users at 5 years (P=0.02) and 7.4% versus 5.4% at 13 years (not significant); combined low and borderline-low B12 was 20.3% versus 15.6% at 13 years (P=0.02), and the odds of deficiency rose 13% per year of metformin use (OR 1.13, 95% CI 1.06 to 1.20) [9].

Bottom line

The label's schedule starts at 500 mg twice daily or 850 mg once daily and climbs to 2,550 mg a day [2]. The smallest dose in the placebo-controlled dose-response trial was 500 mg a day, and benefit was clearest as the dose rose toward 2,000 mg [1].

Learn more about metformin.