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  • Writer's pictureGraham Exelby

Doxycycline and Metformin in Covid Management

- Repurposing old medication

Metformin and Doxycycline are “old” medications that have shown to have potential value in management of Covid. Doxycycline in particular is proving to be helpful in the resistant “Long Covids.” In particular its ability to block TLR4 and reduce cytokines IL-6 and TNFa makes this a potentially valuable tool in Long Covid as well as acute Covid management, especially when combined with Zinc.


Metformin is a mainstay in the management of type 2 diabetes was first introduced in France in 1957, developed from a herb Galega officalis. From mediaeval times it had been used to treat what we would now know as diabetes. Unlike insulin and sulphonureas, it primarily reduced glucose release from the liver. It was rediscovered in 1995 when it was found to reduce the risk of myocardial infarction and overall mortality from all causes.(4)

"A study released in 2023 by Bramante, Buse, Liebovitz, et al (1) showed that Metformin (Diabex) lowers the risk of getting Covid by 40%, and if this was started less than 4 days after Covid symptoms started, the risk of Long Covid was decreased by 63%. The metformin dose was titrated over 6 days: 500 mg on day 1, 500 mg twice daily on days 2–5, then 500 mg in the morning and 1000 mg in the evening up to day 14.(2)

TLR4 is a receptor involved in the innate immune response and is responsible for recognizing various pathogen-associated molecular patterns (PAMPs) and damage-associated molecular patterns (DAMPs). Activation of TLR4 triggers a signalling cascade that leads to the production of pro-inflammatory cytokines and the activation of immune cells.

Studies have shown that metformin can inhibit TLR4 signalling, leading to decreased inflammation and improved insulin sensitivity.

The key findings are:

  1. Inhibition of TLR4 Expression: Metformin has been shown to reduce the expression of TLR4 in various cell types, including macrophages and adipocytes. This downregulation of TLR4 limits its ability to recognize inflammatory stimuli and attenuates the inflammatory response.

  2. Suppression of TLR4 Signalling Pathways: Metformin has been found to inhibit the activation of downstream signalling pathways associated with TLR4, such as the nuclear factor-kappa B (NF-κB) pathway. NF-κB is a transcription factor that plays a crucial role in the production of pro-inflammatory cytokines. By suppressing NF-κB activation, metformin reduces the release of these inflammatory mediators. (24)(25(26)(27)

  3. Modulation of Gut Microbiota: Metformin has also been found to alter the composition of the gut microbiota, which can indirectly affect TLR4 signalling. Dysbiosis of the gut microbiota has been linked to TLR4 activation and chronic inflammation. Metformin's impact on the gut microbiota may help restore a healthier microbial balance and reduce TLR4-mediated inflammation.

Metformin (Diabex) activates AMPK , a central regulator of energy homeostasis, acts as a signal integration platform to maintain mitochondrial health. Cells constantly adapt their metabolism to meet their energy needs and respond to nutrient availability. Under conditions of low energy, AMPK phosphorylates specific enzymes and growth control nodes to increase ATP generation and decrease ATP consumption. AMP-activated protein kinase (AMPK) is a highly conserved sensor of low intracellular ATP levels that is rapidly activated after nearly all mitochondrial stresses, even those that do not disrupt the mitochondrial membrane potential.(2)"

Figure 2: Regulation and Function of AMPK in Physiology and Diseases

Metabolic functions of AMPK. A schematic summarizing the mechanisms underlying AMPK-induced regulation of diverse metabolic pathways. Arrow indicates activation, and bar-headed line indicates inhibition (see text for details).(3)

Source: Jeon, SM. Regulation and function of AMPK in physiology and diseases. Exp Mol Med 48, e245 (2016). (3)

Doxycycline (with Zinc):

“Doxycycline is a member of the tetracycline class of antibiotics and has been used clinically for more than 40 years. It is bacteriostatic and acts via the inhibition of bacterial ribosomes. It is generally given at a dose of 100-mg daily or twice daily. It is well absorbed and has generally good tissue penetration. The serum half-life is 18-22 hours and dosage does not need to be adjusted in the presence of renal or hepatic impairment.”

“Major side effects are gastro-intestinal and dermatological and it is generally contra-indicated in pregnancy or childhood because of concerns about discolouration of developing teeth and potential effects on growing bones. Drug interactions are not common although can occur with the concomitant use of methotrexate and the oral contraceptive pill, and its absorption can be reduced by the co-administration with some antacids and iron preparations. It has activity against many organisms, including Gram-positives, Gram-negatives and atypical bacteria. In addition, it appears to have some potentially clinically useful anti-inflammatory properties.”(5)

Penetration occurs in body fluids and tissues and high levels are detected in a range of tissues including lymphatic fluid, peritoneal fluid, colonic tissue, prostate tissue, and breast milk. It also penetrates into cerebrospinal fluid. Concentrations are highest in excretory organs including the biliary system. There is poor concentration in saliva and sputum.” (5)

Doxycycline has been approved for the prevention or treatment of specific conditions within each of the following categories: rickettsial infections, sexually transmitted infections, respiratory tract infections, bacterial infections, Lyme disease, ophthalmic infections, anthrax, acute intestinal amebiasis, traveller’s diarrhea, and severe acne. It has also been investigated as a treatment for specific cancers because some studies suggest that doxycycline can inhibit cell proliferation and invasion and also induce apoptosis and block the gap phase (in which a cancerous cell grows and prepares to synthesize DNA)(6)

Trials by Stambouli et al (7) found the combination of doxycycline and Zinc together provided a protective and curative effect against Covid, at dosages of doxycycline (100 mg/ day) and zinc (15 mg/ day) combined orally for 6 weeks.

Zinc has antiviral effects against certain viruses. “Zinc also has a beneficial role in protecting against COVID-19 infection. Many reports have shown that zinc can inhibit the enzymatic activity and replication of SARS-CoV-2’s ribonucleic acid polymerase and can inhibit angiotensin-converting enzyme activity. It was suggested that zinc can prevent fusion with the host membrane, decrease the viral polymerase function, impair protein translation and processing, block viral particle release, and destabilize the viral envelope.”(7)

Matrix metalloproteinases (MMPs) are a family of extracellular proteinases. “In the lung, essentially all cell types, including epithelial, interstitial, vascular, and inflammatory cells, produce MMPs. MMPs act on a variety of extracellular proteins, such as cytokines, chemokines, antimicrobial peptides, and other proteins, that regulate varied aspects of inflammation and immunity.”(8)

Coronaviruses exploit MMPs for a range of activities -replication, cell infection, and survival. Doxycycline may have an antiviral effect on SARS-CoV-2 by inhibiting MMPs. “Researchers have suggested that doxycycline may delay COVID-19 progression through anti-inflammatory activities, including viruses that regulate the NF- κB pathway (nuclear factor kappa-light-chain-enhancer of activated B cells) and inhibit proinflammatory cytokine levels (interleukin 6 (IL-6), interleukin 1β (IL-1β), TNFα tumour necrosis factors (TNFα)) during acute respiratory distress syndrome (ARDS) in severely ill patients with COVID-19.”(7)

The potential effect of combined therapy may be explained by the ability of doxycycline to catalyze Zn2+ ions, which are required for the activity of MMPs, independently of its antimicrobial properties.

Doxycycline is the most potent tetracycline derivative inhibitor of MMPs, even at low doses (25 mg).

It may act as an ionophore by increasing intracellular zinc concentrations, suppressing viral replication, and strengthening the immune system.

Doxycycline has also been shown to reduce TLR2 and TLR4 activity and consequent reduction in IL-6 and TNFa levels.(9) Key findings regarding the potential effects of doxycycline on TLR4 signalling:

  1. Inhibition of TLR4 Expression: Doxycycline has been shown to reduce the expression of TLR4 in various cell types, including macrophages and lung epithelial cells. This downregulation of TLR4 limits its ability to recognize inflammatory stimuli and may attenuate the inflammatory response.

  2. Suppression of TLR4-Mediated Inflammation: Doxycycline has been found to inhibit the activation of downstream signalling pathways associated with TLR4, such as the nuclear factor-kappa B (NF-κB) pathway. By suppressing NF-κB activation, doxycycline can reduce the production of pro-inflammatory cytokines and dampen the inflammatory response.

  3. Modulation of TLR4-Related Diseases: Studies have suggested that doxycycline's effects on TLR4 signalling may have therapeutic implications for various TLR4-related diseases. For example, in animal models of acute lung injury, doxycycline treatment was found to reduce lung inflammation and improve overall lung function, potentially through its modulation of TLR4 signalling.

Alexpandi,R et al (23) showed doxycycline inhibited the viral load of strain-dependent SARS-Co in hopspitalized patients by inhibiting viral replication. Dorobisz et al (10) describe that “due to its immunomodulatory, anti-inflammatory, cardioprotective and antiviral effects, it (doxycycline) seems to be an ideal drug for patients with mild, moderate and severe type of COVID-19.

Doxycycline reduces DPP4 expression by blocking the NF-kB pathway, and consequently blocking the penetration of the virus into cells. (17)(18)

Bonzano et al (11) confirmed that doxycycline at a dose of 200 mg daily in patients with COVID-19 administered for 8 days improved the sense of smell. Mostafa (12) confirmed that doxycycline is effective in reducing ARDS induced by COVID-19 and cytokine storms. This is related to the easy penetration of tetracyclines into the pulmonary alveoli due to lipophilicity.

COVID-19 induces the proliferation of mast cells in the airway mucosa, (14) activates the NF-kB pathway,(15) thus increasing the inflammatory response. COVID-19 is also associated with the activation of IL-1b, TNF and IL-6. Mosquera-Sulbaran and Hernandez-Fonseca (13) found that doxycycline reduces virus replication by inhibiting its penetration into cultured cells.

Doxycycline also has a cardioprotective effect. It regulates the MMP-2 pathway, protecting the systolic function of the heart.(22)(16) As a result, it inhibits arrhythmias caused by myocardial ischaemia.”(10)

Doxycycline blocks IL-6, MMPs, and especially MMP-9. MMP-9 is necessary for the penetration of the virus into the cell.(17) IL-6 and MMPs are cytokine storm regulators often associated with severe viral pneumonia. (19)(20) In vivo inhibition of CD14 +/EMMPRIN by doxycycline was found, which may inhibit penetration of SARS-CoV-2 into T cells. (21)


1. Bramante CT, Buse JB, Liebovitz DM, et al. Outpatient treatment of COVID-19 and incidence of post-COVID-19 condition over 10 months (COVIDOUT): a multicentre, randomised, quadruple-blind, parallel-group, phase 3 trial. Lancet Infect Dis 2023; published online June 8. S1473-3099(23)00299-2.

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