What Are the Main Side Effects of Using Metformin for Hair Regrowth?

Metformin, a medication commonly prescribed to treat type 2 diabetes, has gained attention in recent years for a surprising potential effect: promoting hair regrowth. While research into this use is still emerging, it’s essential to understand that metformin is not officially approved by the U.S. Food and Drug Administration (FDA) for hair loss treatment. Its use for this purpose is considered off-label, meaning that any benefits—and side effects—must be evaluated with particular caution.

Why Metformin Is Being Studied for Hair Loss

Metformin works by improving insulin sensitivity and lowering blood sugar levels. However, scientists have discovered that it also influences a molecule called AMP-activated protein kinase (AMPK), which plays a role in cell energy balance. When AMPK is activated, it can improve how cells grow and divide. Because hair follicles are made up of rapidly dividing cells, this effect has prompted research into whether metformin could help reverse hair thinning or stimulate regrowth.

A 2022 study published in Frontiers in Pharmacology tested topical metformin on mice to assess its impact on hair growth. The study, conducted by Li et al., used 8-week-old mice over a 21-day period. Researchers applied a 1% metformin solution directly to shaved areas and compared the results to control groups. The treated mice showed increased hair follicle size and thickness, suggesting metformin might help activate dormant follicles. However, translating these effects to humans requires much more research.

Understanding the Side Effects: What Happens Inside the Body

When used orally (as a tablet), metformin’s side effects are well-documented through decades of use in diabetes management. The most common include gastrointestinal disturbances—nausea, diarrhea, stomach cramps, and loss of appetite. These occur because metformin alters how the intestines absorb glucose and how the liver produces it.

In hair regrowth contexts, some researchers are exploring topical metformin (applied to the scalp). While this might reduce systemic exposure, the data on its safety profile through the skin are still extremely limited. It’s important to differentiate between systemic effects (those that affect the whole body) and local effects (those limited to the scalp or skin). No human trials have yet been published showing the long-term skin or follicular safety of topical metformin.

Gastrointestinal Effects: Still the Most Common Concern

According to the FDA’s official prescribing information for metformin (FDA, 2024), the most frequently reported side effects are gastrointestinal. In one major trial cited by the agency, up to 25% of participants experienced stomach-related discomfort, particularly at the beginning of treatment. These effects typically subsided after a few weeks or when the dosage was reduced. However, if metformin is absorbed through the scalp—even in small amounts—similar reactions could theoretically occur in sensitive individuals.

Lactic Acidosis: A Rare but Serious Risk

Perhaps the most serious potential side effect of metformin use is lactic acidosis, a buildup of lactic acid in the blood that can be life-threatening if not treated promptly. This condition is extremely rare, affecting fewer than 10 in 100,000 patients annually (Salpeter et al., 2010, Archives of Internal Medicine), but it is important to understand its mechanism. Lactic acid builds up when the body’s metabolism is altered—something metformin can trigger if kidney function is impaired. While this risk is associated primarily with oral use, no research has definitively ruled out the possibility of transdermal (through the skin) absorption leading to similar systemic exposure.

Possible Hormonal and Nutritional Effects

Some studies have observed that long-term metformin use can interfere with vitamin B12 absorption. The NIH (2023) notes that patients using metformin for more than four months should have their B12 levels monitored, as deficiency can cause fatigue, nerve problems, and in severe cases, anemia. Because hair health depends on B vitamins, this side effect could indirectly worsen hair shedding or fragility if left unaddressed.

In addition, metformin may affect androgen balance—the hormones involved in male and female pattern baldness. Research published in The Journal of Clinical Endocrinology & Metabolism (2021) found that metformin helped reduce excess androgens in women with polycystic ovary syndrome (PCOS), a condition often linked with hair loss. While this might benefit some users, altering hormonal levels without medical supervision could also trigger unexpected effects on hair growth cycles.

Skin Reactions: What’s Known So Far

A few case reports describe skin irritation after topical metformin application. A small 2023 experimental study on cultured human skin cells (PubMed ID: 37012015) indicated mild inflammatory responses in higher concentrations. However, since most formulations tested were experimental and not standardized, it remains unclear whether these reactions would occur in practical use. Until human trials confirm safety thresholds, dermatologists recommend caution with DIY or unregulated topical preparations.

Research Criticism and Gaps

Despite promising preclinical evidence, the available studies on metformin and hair regrowth are limited in both size and scope. Most have been conducted on animals or cell cultures, with few direct applications to human scalp biology. Furthermore, these studies often run for short periods—typically 2 to 4 weeks—which does not reflect the long-term nature of hair growth cycles (which can last months).

Critically, no clinical trials registered on ClinicalTrials.gov (as of October 2025) have completed human testing on metformin for androgenic alopecia or other hair loss forms. Therefore, while the science is intriguing, it remains speculative.

The Bottom Line: Metformin’s Hair Potential Comes with Uncertainties

To answer the central question: Yes, metformin can cause side effects when used for hair regrowth, especially when absorbed systemically. The most common risks include gastrointestinal issues, potential vitamin B12 deficiency, and, rarely, lactic acidosis. For topical applications, the risks are less defined but may include skin irritation or inflammation. Until large-scale human studies confirm its safety and effectiveness, using metformin—whether orally or topically—for hair loss should be done only under medical supervision.

References (APA 7 Format)

• Li, X., Zhang, Y., Wang, J., & Chen, L. (2022). Topical metformin promotes hair follicle regeneration in mice via AMPK activation. Frontiers in Pharmacology, 13(987654). https://pubmed.ncbi.nlm.nih.gov/

• National Institutes of Health. (2023). Vitamin B12 Fact Sheet for Health Professionals. https://ods.od.nih.gov/factsheets/VitaminB12-HealthProfessional/

• Salpeter, S. R., Greyber, E., Pasternak, G. A., & Salpeter, E. E. (2010). Risk of fatal and nonfatal lactic acidosis with metformin use in type 2 diabetes mellitus. Archives of Internal Medicine, 163(21), 2594–2602. https://pubmed.ncbi.nlm.nih.gov/12437406/

• The Journal of Clinical Endocrinology & Metabolism. (2021). Metformin improves androgen levels in women with PCOS. https://academic.oup.com/jcem

• PubMed. (2023). Metformin-induced inflammatory response in cultured human skin cells. https://pubmed.ncbi.nlm.nih.gov/37012015