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A complete guide to slugging — the skincare technique of applying an occlusive petrolatum layer as the final PM step, the physics of TEWL reduction (petrolatum creates a hydrophobic film reducing transepidermal water vapor flux by 98%), why petrolatum is non-comedogenic despite being oil-based, the evidence from Rawlings 1994 and Draelos 2012 for barrier occlusion, who benefits most (dry skin, eczema, retinoid users, compromised barrier) vs who should avoid it (oily acne-prone skin), and how to layer slugging correctly with actives underneath.
· By MedSpot Editorial · 5 min read
Slugging — applying a thin layer of petrolatum (Vaseline) as the final step in a PM skincare routine — is one of the most evidence-supported barrier-repair techniques in skincare, with a well-understood mechanism and a strong safety profile. Here is the complete guide.
Slugging refers to applying white petrolatum or a petrolatum-based product (Vaseline, Aquaphor, CeraVe Healing Ointment) as the last step in the evening routine, after all serums and moisturizers have been applied. The term comes from the skin's appearance after application — shiny, like a slug's trail.
The product is applied thinly — a film, not a thick coating — over the face and allowed to remain overnight. It is rinsed off in the morning.
The skin loses water continuously through the epidermis by passive diffusion — transepidermal water loss (TEWL). In healthy skin, the stratum corneum's lipid matrix (ceramides, cholesterol, fatty acids) naturally limits this loss. In compromised skin — dry skin, eczema, over-exfoliated skin, retinoid-disrupted skin — this lipid barrier is incomplete and TEWL increases.
Petrolatum's mechanism:
This is pure physical occlusion — petrolatum does not penetrate the skin, does not moisturize the skin (it contains no water), and does not provide humectant function. It simply prevents water that is already in the skin from escaping.
The skin's barrier lipids (ceramides, fatty acids) are synthesized in the stratum granulosum and secreted into the stratum corneum. This process requires normal transepidermal water activity — when the skin is severely dehydrated (high TEWL), synthesis is impaired. Occlusion restores the water activity needed for lamellar body lipid secretion and barrier reconstruction.
Rawlings AV, Corcuff P, Agache P, Prunieras M. (1994). The effect of occlusion on stratum corneum lipid composition. Acta Dermato-Venereologica, 74(2), 88–92. Demonstrated that occlusion accelerates stratum corneum lipid (ceramide) replenishment after experimental barrier disruption — the biophysical basis for using petrolatum in eczema and dry skin management.
Petrolatum is oil-based and derived from petroleum — both facts lead many people to assume it will cause breakouts. The evidence consistently shows otherwise:
Petrolatum does not penetrate the follicular canal: Its large hydrophobic hydrocarbon chains cannot dissolve into the sebum-filled follicle. It remains as a surface film — it does not occlude pores in the way that small lipophilic molecules do.
Comedogenicity testing: Petrolatum consistently scores 0 (non-comedogenic) on the rabbit ear assay and is rated non-comedogenic in human studies. It is used in prescription wound care and post-procedure protocols specifically because it does not interfere with follicular healing.
The practical confirmation: Dermatologists have used petrolatum on post-procedure skin (after laser, peels, microneedling) for decades without comedone induction. Its occlusive film is on the surface, not in the follicle.
Dry skin: Chronic dry skin with high baseline TEWL — slugging dramatically reduces overnight water loss and allows the skin to wake up with visibly improved plumpness and smoothness.
Eczema and atopic dermatitis: Petrolatum is a first-line recommendation in atopic dermatitis guidelines. It replaces the deficient barrier lipid function and reduces itch-scratch cycles that perpetuate barrier damage.
Retinoid users: During the retinization period, retinoids transiently impair barrier function. Slugging over retinoid (after the retinoid has absorbed — 20–30 minutes) reduces TEWL and mitigates dryness and peeling.
Over-exfoliated skin: AHA/BHA overuse causing sensitivity, tightness, and redness benefits from a slugging "reset" — 3–7 nights of slugging only (no actives) to allow barrier recovery.
Post-procedure: After laser, chemical peel, or microneedling, the epidermis is temporarily disrupted. Petrolatum-based occlusion is standard clinical practice for post-procedure care.
Cold/dry climates: Low-humidity environments increase TEWL — slugging provides the occlusion that compensates for the environment's drying effect.
Oily and acne-prone skin: While petrolatum is non-comedogenic, the occlusive surface layer can trap heat and create a warm, humid microenvironment on the skin surface — potentially worsening inflammatory acne for some individuals. The evidence for petrolatum causing acne is absent, but the practical experience of some oily-skin patients is negative. Try on a small area first.
Fungal acne (Malassezia folliculitis): An occlusive layer may worsen Malassezia by creating the warm, humid environment it thrives in. Avoid slugging if fungal acne is active.
Over retinoid specifically: After retinoid application, wait 20–30 minutes for absorption, then slug. Do not apply petrolatum before the retinoid — it will prevent absorption.
| Occlusive | TEWL Reduction | Notes |
|---|---|---|
| White petrolatum (Vaseline) | ~98% | Gold standard; cheapest |
| Aquaphor Healing Ointment | ~85% | Petrolatum + lanolin + glycerin; slightly more moisturizing |
| CeraVe Healing Ointment | ~95% | Petrolatum + ceramides; combination barrier support |
| Squalane | ~50% | Lighter; non-comedogenic; less effective occlusion |
| Shea butter | ~40% | Moderate occlusion; contains oleic acid (comedogenic for some) |
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