What Is Slugging? The Science Behind the Petrolatum Overnight Technique

Slugging is one of the few viral skincare trends that is actually grounded in dermatological science rather than invented by an algorithm. The technique — applying a thin layer of petrolatum as the absolute final step of a PM skincare routine — is not new. Dermatologists have used occlusive dressings and petroleum jelly for wound healing and barrier repair for over a century. What changed is that the broader skincare community discovered it, named it, and started applying the logic systematically to nightly routines.

The Science of Occlusion: Why Petrolatum Works

Petrolatum (petroleum jelly, Vaseline) is a semi-solid mixture of hydrocarbons derived from petroleum refining. Its mechanism of action in skincare is purely physical: when applied to the skin surface, it forms a hydrophobic (water-repelling) film that dramatically slows transepidermal water loss (TEWL) — the passive diffusion of water vapour through the skin and into the air.

TEWL is one of the primary measurable indicators of skin barrier function. A healthy, intact barrier has low TEWL; a compromised barrier — from over-exfoliation, retinoid adaptation, environmental damage, or genetic barrier dysfunction — has elevated TEWL, which correlates with dryness, sensitivity, and increased reactivity. By applying petrolatum as a final occluding layer overnight, slugging dramatically reduces TEWL during the hours when skin is undergoing repair and regeneration — the most active period of the skin's natural recovery cycle.

Petrolatum is measurably more effective at reducing TEWL than any other commonly used occlusive agent. Studies have shown it reduces TEWL by 98–99% compared to untreated skin — far exceeding the occlusive performance of dimethicone (~20–30%), shea butter (~40%), or lanolin (~65%). This is why it remains the gold standard occlusive in clinical wound care and barrier repair protocols.

Does Petrolatum Clog Pores?

This is the most persistent concern about petrolatum use on the face, and the evidence consistently shows it to be unfounded at normal use quantities. Petrolatum consistently scores zero or near-zero on comedogenicity testing and is classified as non-comedogenic by the American Academy of Dermatology. Despite its thick texture, its molecular structure is too large to penetrate the follicle and create a true comedone-forming obstruction.

The nuance: applying a thick layer of petrolatum over pores that already contain clogged, sebum-filled debris can trap that material and make existing congestion worse. This is different from petrolatum causing comedones. The practical implication is that slugging is most appropriate for dry and normal skin types with a clean, well-cleansed base — not for actively congested or acne-prone skin with existing blockages.

How to Slug Correctly

Step 1 — Cleanse thoroughly. Double cleanse on any day you have worn SPF or makeup. You are about to apply an occlusive layer that will sit on your skin all night — whatever is beneath it will stay there. Clean skin only.

Step 2 — Complete your full PM routine. Apply all your serums, treatments, and moisturiser as normal. The petrolatum goes over everything else — it does not replace any step. Retinol, niacinamide, peptide serums, and moisturiser all go on first and absorb for a few minutes.

Step 3 — Apply a pea-sized amount of petrolatum. Warm a pea-to-bean sized amount between fingertips until it becomes slightly fluid, then press gently over the face. You do not need a thick coat — a thin, sheer layer is sufficient to create occlusion. Pay particular attention to areas of dryness or barrier compromise.

Step 4 — Sleep. The occlusive effect works passively overnight. Wash it off normally in the morning as part of your AM cleanser step.

What Slugging Does to Active Ingredients Underneath

An important and underappreciated aspect of slugging is that petrolatum applied over active ingredients significantly increases their contact time with the skin and may increase their effective penetration. Under a petrolatum occlusive, the hydration level of the stratum corneum rises, which swells the corneocytes (outer skin cells) and potentially increases the permeability of the barrier to water-soluble ingredients underneath. This is a meaningful effect for actives like niacinamide and hydrating ingredients, but it is also relevant for more potent actives.

For retinol in particular: slugging over retinol significantly increases penetration and therefore both efficacy and potential for irritation. This is not recommended for those still building retinol tolerance — the increased penetration under occlusion can cause more irritation than the retinol alone would produce. Experienced retinol users who can tolerate high concentrations comfortably may find slugging over retinol accelerates their results, but beginners should apply petrolatum separately from retinol or on alternate nights.

For ceramides and barrier-supportive moisturisers: slugging is complementary and enhances their barrier-repair effect. This is the lowest-risk combination and the most widely suitable use of the technique.

Who Should Slug

• Dry and very dry skin types: The most clear-cut beneficiaries. Slugging reliably improves overnight hydration and softness in dry skin and the effects are typically noticeable after a single night.
• Barrier-compromised skin: Skin recovering from over-exfoliation, retinoid adaptation period, eczema flares, or environmental damage responds well to the reduced TEWL that slugging provides. It is one of the core techniques for damaged skin barrier recovery.
• Sensitive and reactive skin: Petrolatum is hypoallergenic, fragrance-free, and extraordinarily well-tolerated. For skin that reacts to almost everything else, pure pharmaceutical-grade petrolatum is one of the safest possible topical options.
• Anyone experiencing very dry winters or air conditioning exposure: Environmental conditions that elevate TEWL are strong situational indicators for slugging even if your skin type does not normally require it.

Who Should Avoid Slugging

• Acne-prone skin with active congestion: The occlusion can trap existing debris and worsen congestion in those with already-blocked pores. A targeted slug on non-congested areas (cheeks, forehead) while avoiding the T-zone is a workable compromise for combination skin.
• Fungal acne (Malassezia folliculitis) prone skin: Heavy occlusion with petrolatum can create the warm, lipid-rich environment that Malassezia yeast thrives in. Avoid if fungal acne is a known concern.
• Retinol beginners: As discussed — the occlusion-enhanced penetration can cause more irritation than expected. Wait until retinol tolerance is established before slugging over it.

Petrolatum Alternatives for Slugging

Pure petrolatum (Vaseline, pharmaceutical-grade petroleum jelly) is the reference product for slugging. Some alternatives that achieve a similar but less potent occlusive effect include aquaphor (petrolatum plus humectants and emollients — a good all-rounder), CeraVe Healing Ointment (petrolatum plus ceramides), and pure lanolin. None of these achieve the same level of TEWL reduction as pure petrolatum, but they are more cosmetically elegant and better tolerated on some skin types.

Squalane is sometimes suggested as an alternative, but as a lightweight emollient rather than a true occlusive, it provides significantly less barrier-sealing effect. It is an excellent skincare ingredient in its own right, but not a functional substitute for petrolatum in a slugging protocol.