The Hair Growth Cycle Explained (2026): Anagen, Catagen, Telogen, Exogen — and Why Every Treatment Targets One of Them
📌 TL;DR
- Hair grows in cycles, not continuously. Each follicle goes through anagen (growth, 2–6 years on scalp), catagen (regression, 2–3 weeks), telogen (rest, 2–4 months), and exogen (release of the old hair). The percentages and durations are species- and site-specific.
- About 85–90% of healthy scalp follicles are in anagen at any moment, 1–2% in catagen, and 10–15% in telogen/exogen. A normal scalp sheds roughly 50–100 hairs a day — exogen events scattered across the year.
- Treatments work by manipulating phase transitions. Minoxidil pushes telogen → anagen. Finasteride/DHT-blockers extend anagen by removing the miniaturisation signal. Telogen effluvium synchronises mass telogen entry. AGA is cycle compression — anagen shortens with each round.
- Most treatment timelines are dictated by cycle biology, not drug pharmacology. Why minoxidil takes 4–6 months to show results, why finasteride needs 6–12 months, why postpartum recovery takes 8–12 months — they all reflect how long one full cycle takes.
The Hair Growth Cycle Explained (2026)
Last updated: May 2026 | Written by RK
Almost every hair-loss conversation eventually comes back to the same question: why is this taking so long? Six months on minoxidil, three months on finasteride, eight months of postpartum shedding waiting to stop. The reason is always the same — the timescale isn’t set by the drug, it’s set by the cycle the drug is trying to influence.
This is the foundational article. Once you understand the four phases and the signaling that governs them, the rest of the hair-loss landscape — minoxidil, DHT blockers, telogen effluvium, female pattern hair loss, microneedling — clicks into place as different ways of nudging the same machine.
Why the cycle is the right mental model
A single follicle is not constantly producing hair. It runs through a programmed sequence — produce, regress, rest, release — and starts again. Different follicles on the same scalp are in different phases at any moment, so the scalp as a whole looks like a steady population even though individual follicles are highly dynamic.
The two facts that make the cycle the right unit of analysis:
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Every hair-loss condition is a cycle disturbance. Androgenetic alopecia is anagen shortening each round. Telogen effluvium is mass premature telogen entry. Alopecia areata is anagen disruption by autoimmune attack. Even chemotherapy-induced loss is anagen-effluvium — drug damage to actively growing follicles.
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Every hair-loss treatment manipulates a phase transition. Minoxidil shifts late-telogen follicles into early anagen. Finasteride extends anagen by removing the DHT miniaturisation signal. Spironolactone does the same via androgen receptor blockade. Microneedling speeds the absorption of whatever you put on top of it.
If you don’t know the phases, treatment outcomes feel arbitrary. Once you do, the timing and expected response of each drug stops being magic.
The four phases — in numbers
The classical “three-phase” model (anagen, catagen, telogen) was the standard for decades, with exogen treated as part of telogen. Modern dermatology increasingly recognises exogen as a separately controlled phase [3]. The scheme below uses the four-phase model.
Anagen — the productive phase
Anagen is when the follicle does actual work. Matrix cells at the base divide rapidly, melanocytes pigment the emerging shaft, and the lower follicle reaches deep into the dermis. The duration of anagen is genetically programmed and site-specific — it’s the single biggest determinant of maximum hair length.
For scalp follicles, anagen is exceptionally long (2–6 years), which is why scalp hair grows long without an obvious upper limit for most people. Eyebrows, eyelashes, and body hair have anagen phases measured in weeks, capping their growth at short lengths.
About 85–90% of healthy scalp follicles are in anagen at any given moment [1][2]. That percentage drops in AGA (because anagen shortens and follicles cycle faster), in active telogen effluvium (because the proportion in telogen briefly spikes), and during chemotherapy (because anagen follicles are selectively damaged).
Catagen — the demolition phase
Catagen is a tightly programmed apoptotic event. The lower follicle (everything beneath the bulge) regresses upward — about two-thirds of the follicle disassembles over 2–3 weeks. The hair shaft completes itself into a club shape and detaches from the dermal papilla.
Only 1–2% of follicles are in catagen at any moment because the phase is so brief. This is also why catagen is the hardest phase to study in clinical samples.
Telogen — the holding phase
Telogen is restful from the follicle’s perspective, but it’s a misnomer to think of it as inactive. The club hair sits in the follicle, anchored just enough to stay put, while the bulge stem cells receive signals that determine when to start a new anagen.
Telogen on the scalp lasts about 2–4 months. The crucial implication: when something synchronously pushes a population of follicles into telogen — pregnancy ending, severe illness, crash diet, antibiotics, surgery — the visible shedding doesn’t appear immediately. It appears 2–4 months later when those follicles transition into exogen all at once. This is why postpartum hair loss starts around month 3, not month 0.
Exogen — the active release
For decades exogen was treated as a “passive falling out” at the end of telogen, but modern molecular work shows it’s actively regulated [3]. The mechanism that releases the club hair is distinct from the mechanism that initiates the next anagen. They overlap in timing but are decoupled in control.
The fact that exogen is separately controlled also explains some apparently odd findings: cases where new anagen has clearly started but the old club hair is still in place, cases where follicles enter kenogen (an empty follicle state with no club hair and no new growth), and inter-individual variation in shedding amount that doesn’t track perfectly with telogen percentages.
What controls phase transitions
The molecular control of hair cycling is now reasonably well-mapped, even if not fully understood. The signaling crosstalk that runs the cycle is dominated by four families: Wnt, BMP, Shh, and FGF — with androgen and estrogen pathways layered on top in regions where they matter [2].
The high-level pattern: Wnt and BMP are antagonists running the cycle. Wnt drives growth; BMP drives rest and regression. The follicle moves between phases as these two signals oscillate. Most drugs and conditions exert their effect by tilting this balance — directly or indirectly.
What disrupts the cycle — and which phase each disruption targets
This is the practical table. Once you know which phase a condition or treatment touches, the timeline and the expected response stop being mysterious.
The most useful single insight from this table: AGA and TE look superficially similar (both are hair shedding), but they’re opposite ends of the cycle problem. AGA is anagen failing to be long enough; TE is anagen being terminated prematurely. AGA needs years of treatment; TE needs months of patience. Confusing the two leads to wrong treatment.
How dermatologists measure the cycle
You can’t see phase transitions with the naked eye. Three tools used in clinic and research:
Phototrichogram (clinical standard) A small scalp area is shaved or trimmed, photographed at day 0 and again 2–3 days later. Hairs that grew between the two images are anagen; hairs that didn’t are telogen. Quantitative, reproducible, useful for tracking treatment response over 6+ months. Modern variants (TrichoScan) automate the analysis with software.
Trichogram (pluck-based, less common) A small sample of hairs is plucked and examined under the microscope. Anagen hairs have a pigmented, intact bulb with sheath; telogen hairs have the white club shape. Quantitative but uncomfortable.
Hair pull test (bedside screen) The dermatologist grasps a tuft of about 60 hairs and tugs gently. More than 6–10 hairs released is considered positive — suggesting increased telogen population or active TE. Quick, useful as a screen, not a diagnostic on its own.
For most people deciding whether their treatment is working, the practical version is standardised photos every 3 months — phone camera, same lighting, same time of day, same hair position. Crown-down, frontal, and one side profile. This is the patient-side substitute for a phototrichogram and the single most useful thing you can do for yourself.
Putting it together — your treatment is targeting which phase?
Once you map your situation to a phase, expected timeline and next-best move usually become obvious.
Recent heavy shedding without obvious thinning
Likely telogen effluvium. Synchronised exogen wave from a trigger ~2–4 months ago (illness, surgery, stress, postpartum, weight loss). Expect resolution in 6–12 months. Drugs usually unnecessary; address the trigger.
Slow density loss over years, pattern-based
Likely AGA / FPHL — anagen compression. Treatment goal: re-extend anagen. Minoxidil pushes the cycle; DHT blockers remove the shortening signal. Timeline 6–12 months minimum to see response.
On minoxidil, month 1–2, shedding more than baseline
The expected dread shed — forced telogen → anagen transition releases old club hairs. Mechanically correct, will resolve by month 3–4 as new anagen growth dominates. Do not stop the drug here.
Six months into any treatment, deciding if it’s working
Six months ≈ one cycle’s worth of evidence. Compare photos in consistent lighting, same hair position, crown-down. If density is stable or improving, continue. If still actively losing, escalate (combine treatments, add microneedling, see derm).
What to read next
- DHT and Hair Loss — How It Works — the molecular driver of anagen shortening in AGA.
- Minoxidil for Hair Loss — The Complete Guide (2026) — the canonical cycle-pushing drug.
- Microneedling for Hair Loss — The Dermaroller Guide (2026) — drug delivery + wound-healing growth factors.
- Postpartum Hair Loss Recovery Guide (2026) — the textbook example of synchronised telogen entry.
- Female Pattern Hair Loss — The Complete Guide (2026) — anagen compression in the female pattern.
Citations
[1] Stenn KS, Paus R. “Controls of hair follicle cycling.” Physiol Rev 2001;81(1):449–494. (PMID 11152763 — the foundational review)
[4] Plikus MV, Mayer JA, de la Cruz D, et al. “Cyclic dermal BMP signalling regulates stem cell activation during hair regeneration.” Nature 2008;451(7176):340–344. (Wnt/BMP coupled oscillator — foundational cycle biology paper)
[5] Trüeb RM. “Discovering the Secrets of the Hair Cycle: Practical Implications for Trichology.” Various reviews of phototrichogram and clinical hair cycle measurement.
Disclaimer. This article is educational, summarising published hair biology. It is not medical advice. Drug behaviour and timelines vary between individuals. If you’re losing hair in an unusual pattern, see a dermatologist for diagnosis before starting any treatment.
