Why Metformin is Contraindicated in Renal Disease

1. Pharmacokinetic Basis - Drug Accumulation

• Not metabolized - excreted entirely unchanged in the urine
• Not protein-bound - freely circulates in plasma
• Half-life is 1.5-5 hours under normal renal function
• Renal uptake is mediated by organic cation transporter type 2 (OCT2), and export into urine is via MATE1/2 (multidrug and toxin extrusion proteins)

• Katzung's Basic and Clinical Pharmacology, 16e
• Goodman & Gilman's The Pharmacological Basis of Therapeutics

2. The Core Risk - Lactic Acidosis

• Katzung: "As a consequence of metformin's blockade of gluconeogenesis, the drug may impair the hepatic metabolism of lactic acid. In patients with renal insufficiency, the biguanide accumulates and thereby increases the risk of lactic acidosis, which appears to be a dose-related complication."
• Miller's Anesthesia, 10e: "Renal impairment appears to be a significant risk factor [for MALA]."

3. eGFR-Based Thresholds (Current Guidelines)

• Katzung: "Metformin can be safely used in patients with estimated glomerular filtration rates (eGFR) between 60 and 45 mL/min per 1.73 m². It can be used cautiously in patients with stable eGFR between 45 and 30 mL/min per 1.73 m². It is contraindicated if the eGFR is less than 30 mL/min per 1.73 m²."

4. Additional Risk Factors That Compound the Problem

• Congestive heart failure (reduced renal perfusion + tissue hypoxia)
• Sepsis (hypoperfusion, elevated lactate)
• Decompensated liver failure (reduced lactate clearance)
• Contrast media (risk of acute kidney injury post-procedure)
• NSAIDs (reduce renal blood flow and GFR, raising metformin concentration)

5. Is the Causal Link Certain?

Summary

Metformin Contraindicated in Renal Disease - Key Points

• Excreted unchanged by kidneys (not metabolized)
• Not protein-bound → freely accumulates
• In renal failure → drug accumulates → toxic plasma levels

• Metformin blocks hepatic gluconeogenesis → liver cannot clear lactate
• Inhibits mitochondrial Complex I → shifts metabolism to anaerobic → more lactate produced
• Result = Metformin-Associated Lactic Acidosis (MALA) - high mortality

• eGFR > 60 → Safe
• eGFR 45-60 → Caution + monitor
• eGFR 30-45 → Use cautiously (stable CKD only)
• eGFR < 30 → Contraindicated

• Decompensated liver failure (impaired lactate clearance)
• Heart failure (tissue hypoxia + poor renal perfusion)
• Sepsis (hypoperfusion)
• Pre-contrast media administration (risk of acute kidney injury)
• NSAIDs co-use (reduce GFR → raise metformin levels)

Metformin accumulates in renal failure → blocks lactate clearance + promotes lactate production → lactic acidosis

Metformin in Renal Disease - With Enzymes (Exam Focus)

• Activates AMPK (AMP-activated protein kinase) → suppresses hepatic gluconeogenesis
• Inhibits mitochondrial Complex I (NADH dehydrogenase) → blocks oxidative phosphorylation → anaerobic glycolysis → ↑ lactate
• Inhibits mitochondrial glycerol-3-phosphate dehydrogenase (mGPD) → alters hepatic redox state (↑ NADH/NAD⁺ ratio) → lactate cannot be converted to pyruvate

• Enters hepatocytes via OCT1 (Organic Cation Transporter 1)
• Renal uptake via OCT2 (Organic Cation Transporter 2)
• Excreted into urine via MATE1/MATE2 (Multidrug And Toxin Extrusion proteins)
• In renal failure → OCT2/MATE1/2 fail → drug trapped → accumulation

• < 30 mL/min/1.73 m² → Contraindicated
• 30-45 → Cautious use only
• 45-60 → Monitor closely

• 60 → Safe

Metformin inhibits Complex I and mGPD → ↑ lactate; in renal failure, OCT2/MATE transport fails → accumulation → fatal lactic acidosis

Biguanides Comparison: Metformin vs Phenformin vs Buformin

Quick Status

Detailed Comparison (Exam Points)

Key Enzyme - Why Phenformin is Worse (Exam Critical)

• All biguanides inhibit mitochondrial Complex I (NADH dehydrogenase)
• Phenformin/buformin = highly lipid soluble → easily cross mitochondrial membrane → bind Complex I more avidly → greater inhibition → more anaerobic glycolysis → far more lactate
• Metformin = low lipid solubility → limited mitochondrial membrane crossing → weaker Complex I binding → much lower lactic acidosis risk

"Metformin has a markedly lower incidence of lactic acidosis than phenformin, possibly because it is less lipid soluble and thus has limited ability to cross the mitochondrial membrane and bind to mitochondrial complex." - NKF Primer on Kidney Diseases, 8e

Why Phenformin Was Withdrawn - Key Facts

• Lactic acidosis incidence with phenformin: ~40-64 cases per 100,000 patient-years
• Lactic acidosis with metformin: ~3-5 cases per 100,000 patient-years (much rarer)
• Phenformin withdrawn from USA in 1976, UK/Europe by 1978
• Buformin withdrawn around the same period for identical reasons

Common Mechanism (All Biguanides)

1. Activate AMPK → suppress gluconeogenesis
2. Inhibit Complex I (NADH dehydrogenase) → reduce ATP → impair oxidative phosphorylation
3. Inhibit mGPD (mitochondrial glycerol-3-phosphate dehydrogenase) → alter NADH/NAD⁺ ratio → impair lactate clearance

One-liner for Exam

Phenformin and buformin are highly lipid soluble → cross mitochondrial membrane freely → inhibit Complex I severely → cause fatal lactic acidosis → withdrawn in 1970s; Metformin's low lipid solubility limits this → still in use today

MOA of Combined Oral Contraceptive Pills (COCP)

Components

• Estrogen - Ethinyl estradiol (EE), 20-35 µg
• Progestin - Levonorgestrel, norethindrone, drospirenone, desogestrel, norgestimate (varies by formulation)

Mechanisms (Multiple Levels - All Exam-Important)

1. Suppression of Ovulation (PRIMARY mechanism)

• Continuous estrogen → negative feedback on hypothalamus → ↓ GnRH pulsatility
• → ↓ FSH secretion from anterior pituitary
• → No follicular development → no dominant follicle formed
• → No estrogen surge → no LH surge → ovulation blocked

• Suppresses LH secretion directly at pituitary level
• Prevents the mid-cycle LH surge → ovulation cannot occur

"Negative feedback from continuous estrogen administration decreases FSH secretion, follicular development, and formation of a dominant follicle. Continuous progestin suppresses LH secretion and inhibits ovulation." - Harrison's Principles, 22e

2. Thickening of Cervical Mucus (Progestin effect)

• Progestin → cervical mucus becomes thick, viscous, scanty
• Hostile to sperm penetration
• Sperm cannot reach the egg even if ovulation occurs (backup mechanism)

3. Endometrial Changes (Progestin effect)

• Progestin → endometrium becomes thin, atrophic, out of phase
• Glands and stroma become unreceptive
• Even if fertilization occurs, implantation is impaired

4. Impaired Tubal Motility (Progestin effect)

• Progestin reduces fallopian tube peristalsis
• Slows ovum transport
• Reduces chance of sperm-ovum meeting

5. Suppression of Ovarian Androgen Synthesis (Estrogen effect - bonus)

• Estrogen → ↑ SHBG (Sex Hormone Binding Globulin) in liver
• More androgens bound → less free testosterone → anti-androgenic effect
• Useful in PCOS, acne, hirsutism

Summary Table

Efficacy

• Failure rate: ~8 pregnancies per 100 women/year (typical use)
• With perfect use: < 1%
• Primary mechanism = ovulation suppression (most reliable)
• Backup mechanisms = cervical mucus + endometrium (if ovulation breakthrough occurs)

One-liner for Exam

COCP: Estrogen suppresses FSH (no follicle) + Progestin suppresses LH (no ovulation) + thickens cervical mucus + renders endometrium hostile + impairs tubal motility

• Harrison's Principles of Internal Medicine, 22e
• Goldman-Cecil Medicine, International Edition

MOA of Progestin-Only Pills (POP) / Minipill

What it Contains

• Only progestin - no estrogen
• Examples: Norethindrone 0.35 mg, Norgestrel 0.075 mg, Drospirenone 4 mg
• Taken continuously (no pill-free days)

Mechanisms (in order of importance)

1. Thickening of Cervical Mucus (PRIMARY & Most Reliable)

• Progestin → acts on cervical epithelium
• Mucus becomes thick, viscous, scanty, and hostile to sperm
• Sperm cannot penetrate the cervix
• This effect begins within 2-4 hours of taking the pill
• Effect wanes after 20-22 hours - that is why the pill must be taken at the same time every day (within a 3-hour window)

"This dose of progestin causes cervical mucus thickening to be impenetrable to sperm." - Textbook of Family Medicine, 9e

2. Endometrial Changes (Important backup mechanism)

• Progestin → thin, atrophic endometrium
• Glandular atrophy + stromal decidualization
• Endometrium becomes out of phase and unreceptive
• Even if ovulation and fertilization occur, implantation is prevented

"Thinning of the endometrium to reject implantation." - Textbook of Family Medicine, 9e

3. Suppression of Ovulation (Inconsistent - KEY exam point)

• POP does NOT reliably suppress ovulation - this is the major difference from COCP
• Ovulation is suppressed in only ~50% of cycles (low-dose POPs)
• Progestin partially blunts the LH surge but not consistently enough to prevent all ovulations
• Higher-dose POPs (e.g., desogestrel 75 µg) suppress ovulation more consistently (~97%)
• This is why cervical mucus effect is considered the primary contraceptive mechanism

4. Impaired Tubal Motility

• Progestin reduces fallopian tube peristalsis and ciliary activity
• Slows ovum and sperm transport
• Reduces chance of fertilization even if ovulation occurs

Endometrial Effects - Detailed (Exam Focus)

• Irregular/unpredictable bleeding (most common side effect of POP)
• Spotting, amenorrhea, or erratic cycles - due to atrophic unstable endometrium
• No proper "withdrawal bleed" unlike COCP

POP vs COCP - Key Differences (Exam Table)

Who Gets POP (Indications - Exam)

• Breastfeeding mothers (postpartum) - estrogen reduces prolactin/milk
• Women with DVT/thrombophilia (no estrogen → no clotting risk)
• Women with hypertension, migraine with aura
• Smokers > 35 years
• Women who cannot tolerate estrogen

One-liner for Exam

POP primarily works by making cervical mucus impenetrable to sperm; also thins endometrium to prevent implantation; ovulation suppression is inconsistent - must be taken at same time daily within 3-hour window

• Textbook of Family Medicine, 9e
• Harrison's Principles of Internal Medicine, 22e
• Goldman-Cecil Medicine

Why Biphasic Pill Has Reduced Progestin Dose (in First Half)

The Core Reason - Mimicking the Natural Menstrual Cycle

"Biphasic and triphasic preparations provide two or three different pills containing varying amounts of active ingredients, to be taken at different times during the 21-day cycle. This reduces the total amount of steroids administered and more closely approximates the estrogen-to-progestin ratios that occur during the menstrual cycle."

• Goodman & Gilman's Pharmacological Basis of Therapeutics

What Happens Naturally in the Menstrual Cycle

• In the first half of the cycle, the natural body has very little progesterone
• Progesterone only rises significantly after ovulation (from corpus luteum)

How Biphasic Pill Mimics This

• Phase 1: Low progestin mirrors the natural low-progesterone follicular phase
• Phase 2: Higher progestin mirrors the natural post-ovulation luteal phase rise

Reasons for Reducing Progestin in Phase 1 (Exam Points)

• Lower progestin in first 10 days → less cumulative progestin over the month
• Reduces progestin-related side effects (acne, mood changes, weight gain, androgenic effects)

• Low progestin + estrogen in Phase 1 → endometrium proliferates and builds up
• This is needed so that when progestin rises in Phase 2, it can cause proper secretory transformation
• Result: better endometrial stability → less breakthrough bleeding

• Most progestins (levonorgestrel, norethindrone) have androgenic activity
• Keeping Phase 1 dose low → less androgen-related effects (acne, hirsutism, lipid changes)

• Ovulation is still reliably suppressed because even low progestin + estrogen together maintain sufficient negative feedback on the HPO axis
• No compromise in contraceptive reliability

Biphasic vs Monophasic vs Triphasic (Quick Comparison)

One-liner for Exam

Biphasic pill reduces progestin in Phase 1 to mimic the naturally low progesterone of the follicular phase, reduce total steroid load, allow endometrial proliferation, and minimize progestin side effects - without compromising ovulation suppression

• Goodman & Gilman's Pharmacological Basis of Therapeutics

Mastalgia

• From Greek: mastos (breast) + algos (pain)
• Simply means pain in the breast tissue

Types (Exam Points)

Key Facts

• Cyclical mastalgia is the most common type
• Most common in reproductive age women
• Peaks in late luteal phase (days 21-28) → due to ↑ progesterone causing ductal/stromal edema
• Usually bilateral, in upper outer quadrant
• Resolves after menstruation

Clinical Relevance

• A known side effect of COCPs and HRT (hormone-related)
• Also seen with high caffeine intake, high-fat diet
• Must be distinguished from breast lump/carcinoma (cancer is usually painless)