Does slugging (smearing your skin with petroleum jelly) actually work?

Petroleum jelly does indeed seal the skin effectively. It reduces moisture loss through the skin measurably better than plant-based oils or mineral oil: laboratory studies measure 40 to 60% swelling of the outermost skin layer with petroleum jelly, compared with 10 to 20% for those alternatives. That is the mechanism behind slugging: your skin loses less moisture overnight.

When the skin barrier is damaged, for example after irritation or with eczema, petroleum jelly can temporarily cover that barrier and support its recovery. This has been demonstrated in laboratory models, and petroleum jelly is recognised in dermatology as one of the treatment options for eczema. However, large clinical trials in healthy individuals are still lacking.

For dry skin over the longer term, more modern formulas containing ceramides or other lipids that mimic the skin's own structure appear to work better than petroleum jelly alone. Petroleum jelly creates a seal, but it does not add the building blocks the skin itself needs in order to become stronger. If you suffer from structurally dry skin, you will get more benefit from using a ceramide moisturiser, with petroleum jelly applied on top of it if desired.

There is one important safety signal worth mentioning. In infants who received frequent oil baths, the skin looked less dry, but actual transepidermal water loss was temporarily higher. This shows that skin that looks good does not always mean a stronger skin barrier. This research concerned mineral oil in infants, not adults practising slugging, but the principle applies more broadly: the appearance and the function of the skin can diverge.