Acne Explained: Causes, Types, Grades & Symptoms UK
▶ What is acne?
Acne vulgaris is a chronic inflammatory skin condition affecting the pilosebaceous unit — the hair follicle and sebaceous gland. It is caused by four interacting mechanisms: excess sebum production, abnormal skin cell shedding, Cutibacterium acnes bacterial colonisation, and immune-mediated inflammation. It is not caused by poor hygiene. It affects ~95% of people aged 11–30 and can persist into adulthood. Severity is graded Grade 1–4 on the Leeds scale; grade determines treatment under NICE NG198.
Most people have had acne at some point. And most people have been told, at some point, to wash their face more, stress less, or just wait it out. The problem is that acne isn't caused by poor hygiene or lack of patience — it's driven by hormones, genetics, and bacterial colonisation, all interacting in ways that washing or waiting won't address. Understanding what's actually happening under the skin is the first step towards managing it properly.
What Is Acne?
Acne vulgaris develops when hair follicles — most densely distributed on the face, neck, chest, back, and upper arms — become blocked and inflamed. These follicles are part of the pilosebaceous unit: each hair follicle is connected to a sebaceous (oil-producing) gland. When the output of that gland exceeds the capacity of the pore to clear it, a blockage forms. The result depends on whether the blockage is open (a blackhead) or closed (a whitehead), and whether bacteria are present to trigger inflammation.
Acne is not caused by dirty skin or poor hygiene. It is driven by hormonal influences on sebum production, the skin’s cell-shedding behaviour, bacterial colonisation, and the immune response — all of which are biologically determined and not a reflection of personal cleanliness.
What Causes Acne? The Four Mechanisms
Four things have to go wrong at the same time to produce acne. The most effective treatments address more than one of them:
Excess sebum
Androgens (male hormones present in both sexes) stimulate sebaceous glands to produce more oil than the pore can clear. This is why acne peaks at puberty and can flare in women before periods, during pregnancy, and with PCOS.
Abnormal skin cell shedding
Inside the follicle, dead skin cells shed too slowly and accumulate, forming a plug. Retinoids and azelaic acid normalise this process — one reason they are so effective in acne.
Bacterial colonisation
Cutibacterium acnes thrives in the lipid-rich environment of a blocked follicle, multiplying rapidly and triggering the immune system to mount an inflammatory response — producing the redness, swelling, and pain of inflammatory acne.
Inflammation
The immune response to C. acnes generates the visible inflammation of papules and pustules. Anti-inflammatory agents — including azelaic acid and oral tetracyclines — target this step directly.
Types of Acne Lesions
Acne produces two main categories of lesion. Understanding which type you have directly determines the most effective treatment:
| Lesion type | Description | Treatment priority |
|---|---|---|
| Blackhead (open comedone) | Oxidised sebum visible at pore — not dirt | Retinoid to normalise cell shedding |
| Whitehead (closed comedone) | Sebum sealed beneath skin surface | Retinoid or azelaic acid |
| Papule | Small, raised red bump — early inflammatory lesion | Topical antibiotic + benzoyl peroxide |
| Pustule | Papule with white or yellow pus head | Combination gel (Duac, Treclin) |
| Nodule | Large, hard, deep, painful — slow healing | Oral antibiotic + topical retinoid |
| Cyst | Deep pus-filled; highest scarring risk | Dermatology referral; isotretinoin |
How Acne Is Graded: The Leeds Scale
In the UK, clinicians use the Leeds Revised Acne Grading Scale to describe how severe acne is. The grade directly shapes which treatment is recommended under NICE guidelines:
| Leeds Grade | Presentation | NICE first-line treatment |
|---|---|---|
| Grade 0 | No acne | — |
| Grade 1 | Mild comedonal acne — few non-inflammatory lesions | Topical retinoid or azelaic acid |
| Grade 2 | Mild inflammatory acne — some papules and pustules | Benzoyl peroxide + topical antibiotic or retinoid |
| Grade 3 | Moderate inflammatory acne — widespread papules and pustules, some nodules | Oral antibiotic + topical retinoid + benzoyl peroxide |
| Grade 4 | Severe nodulo-cystic acne — extensive nodules, high scarring risk | Dermatology referral; isotretinoin assessment |
Acne vs Rosacea: How to Tell the Difference
Acne and rosacea can look similar — both produce redness and pimple-like spots — but they are distinct conditions with different causes and treatments. Misidentifying rosacea as acne and using comedogenic or sensitising treatments (including high-concentration benzoyl peroxide) can make rosacea significantly worse.
| Acne vulgaris | Papulopustular rosacea | |
|---|---|---|
| Comedones? | Yes — blackheads and whiteheads are hallmarks | No — rosacea does not produce comedones |
| Location | Face, chest, back, shoulders | Central face (cheeks, nose, forehead, chin) |
| Flushing? | Not typically | Common — episodic facial flushing and redness |
| Age of onset | Typically adolescence, but any age | Usually 30+ |
| Triggers | Hormonal changes, diet, medications | UV, alcohol, heat, spicy food, stress |
| First-line topical | Retinoid, benzoyl peroxide, azelaic acid | Metronidazole gel, azelaic acid |
For treatment of rosacea, see: Metronidazole gel for rosacea UK. For treatment of acne, see: Prescription acne treatment online UK.
Acne Triggers and Risk Factors
- Hormonal changes — puberty, pre-menstrual flares, pregnancy, PCOS. Women with jawline and chin spots worsening before periods almost certainly have a hormonal component.
- Genetics — twin studies show high concordance — heritability estimates exceed 80% in some studies. Strong family history significantly raises lifetime risk.
- High-GI diet — refined carbohydrates and sugary drinks raise IGF-1, driving androgen production and sebum output.
- Certain medications — corticosteroids, anabolic steroids, lithium, and some anticonvulsants can worsen or trigger acne.
- Comedogenic products — heavy oil-based cosmetics and skincare can physically block follicles (acne cosmetica).
When to Seek Treatment
See a prescriber if: OTC treatments have not worked after 8 weeks · you have nodular or cystic acne · acne is causing scarring or significant emotional distress · you develop severe, rapidly spreading inflammation or pain. Online consultation provides prescriber assessment without a GP appointment.
For a full guide to all prescription acne treatment options available online in the UK, see: Prescription acne treatment online UK: all options compared.
Struggling with Acne? Get Expert Help Online
GPhC-registered prescribers assess your grade and skin type and recommend clinically proven treatments. No GP appointment needed. GPhC pharmacy #9011198.
Start Your Consultation →Frequently Asked Questions
What causes acne?
Acne is caused by four interacting mechanisms: excess sebum production (driven by androgens), abnormal skin cell shedding inside follicles (creating blockages), Cutibacterium acnes bacterial colonisation of blocked follicles, and an immune-mediated inflammatory response to that bacterial growth. Hormonal fluctuations, genetics, certain medications, and dietary factors can all trigger or worsen acne.
What is acne vulgaris?
Acne vulgaris is the most common form of acne — a chronic inflammatory skin condition affecting the pilosebaceous unit (hair follicle plus sebaceous gland). It produces a range of lesions from non-inflammatory (comedones/blackheads) to inflammatory (papules, pustules) to severe (nodules, cysts). It affects an estimated 95% of people aged 11–30 at some point.
What is the difference between acne and rosacea?
Acne vulgaris is caused by blocked follicles and bacterial colonisation, producing comedones (blackheads and whiteheads) alongside inflammatory lesions. Rosacea is a separate chronic inflammatory condition without comedones, characterised by persistent facial redness, flushing, papules, pustules, and visible blood vessels — often triggered by heat, alcohol, and UV. Acne affects all areas with sebaceous glands; rosacea is typically confined to the central face.
What are the grades of acne on the Leeds scale?
The Leeds Revised Acne Grading Scale (used in UK clinical practice) grades acne from Grade 0 (no acne) to Grade 4 (severe, widespread nodules and cysts). Grade 1 represents mild comedonal acne; Grade 2 mild inflammatory acne; Grade 3 moderate inflammatory acne; Grade 4 severe nodulo-cystic acne. Grade determines treatment — Grade 3–4 typically requires oral antibiotics or specialist referral.
Can diet cause acne?
Growing evidence suggests high-glycaemic diets (refined carbohydrates, sugary drinks) and high dairy intake — particularly skimmed milk — may worsen acne in some people by raising IGF-1 and androgen levels. However, the relationship is not universal. A balanced diet supports skin health, but dietary changes alone rarely clear moderate-to-severe acne.
When should I see a doctor about my acne?
See a doctor if OTC treatments have not worked after 8 weeks, if you have nodular or cystic acne, if acne is causing significant scarring or emotional distress, or if you develop severe inflammation or pain. Access Doctor's online consultations allow prescriber assessment without a GP appointment. GPhC pharmacy #9011198.
References
- NICE. Acne vulgaris: management (NG198). 2021, updated 2025. nice.org.uk/guidance/ng198
- Tan JKL, Bhate K. A global perspective on the epidemiology of acne. Br J Dermatol. 2015;172(S1):3–12.
- O'Brien SC, Lewis JB, Cunliffe WJ. The Leeds revised acne grading system. J Dermatol Treat. 1998;9(4):215–220.
- NHS. Acne. nhs.uk/conditions/acne
- Layton AM et al. British Association of Dermatologists' guidelines for the management of acne vulgaris 2021. Br J Dermatol. 2021;185(6):1012–1030.
Medical disclaimer: This article is for informational purposes only and does not constitute medical advice. Always consult a qualified healthcare professional. Skinoren and Finacea are prescription-only medicines for rosacea in the UK — a clinical consultation is required before they can be dispensed. In a medical emergency, call 999.