Lube osmolality, explained in plain English.

Osmolality is a measure of how many dissolved particles a liquid carries. Your vaginal tissue sits around 260 to 290 mOsm/kg. A lube far above that is hypertonic: it pulls water out of the cells it touches. That is why some lubes sting, why some leave you drier an hour later, and why the WHO says a lubricant should ideally measure 380 mOsm/kg or under.

The number nobody prints.

Walk down the drugstore aisle and every bottle says something: long lasting, pH balanced, gynecologist tested, natural. None of them print osmolality, and osmolality is the number that predicts the most.

Osmolality is concentration. Water with nothing in it is zero. Your blood is about 285 mOsm/kg. Vaginal secretions, 260 to 290. Semen, 250 to 380. A lube in that range is iso-osmolar: it sits on the tissue and does nothing to it chemically. A lube at 2,000 or 6,000 is hypertonic, and osmosis is not optional. Water moves out of the cells toward the lube. Cells shrink, the top layer sloughs, nerve endings get involved.

The sensation is the one people describe as "I think I am allergic to lube." Most of them are not. They are using a lube that is ten to twenty times saltier than they are.

What the WHO actually says.

In 2012 the World Health Organization, UNFPA and FHI360 published an advisory on lubricants for use with condoms. It is the most cited guidance on the subject and it is blunt. Normal vaginal osmolality is 260 to 290. A lubricant should ideally not exceed 380 to minimize epithelial damage. Because most commercial products were far above that, they set an interim procurement ceiling of 1,200 mOsm/kg and told agencies to ask manufacturers for the number.

They also named the cause. Glycerol (glycerin) and propylene glycol are added as humectants, and they drive osmolality. To stay under 1,200, a formula should keep glycerol under 9.9 percent and propylene glycol under 8.3 percent by weight. Warming lubes, which lean hard on propylene glycol, routinely fail.

On pH, the same document says a healthy vagina runs 3.8 to 4.5, the rectum is near 7, and a vaginal lubricant should ideally sit around 4.5.

How the famous brands measure.

Two peer reviewed studies did the lab work. Dezzutti and colleagues measured a shelf of popular lubes in 2012 for a PLoS One paper. Edwards and Panay collected the published values in 2016 for a menopause journal. The numbers that matter:

Product	Osmolality (mOsm/kg)	pH
Sliquid Organics Natural	106	4.0 to 4.4
Good Clean Love Almost Naked	240 to 269	4.7
YES VM moisturizer	250	4.15
Pre-Seed (fertility)	502	7.3
K-Y Jelly	2,007 to 2,510	4.55
Replens moisturizer	1,200 to 2,011	2.5 to 3.5
Astroglide Liquid	6,113 to 8,064	4.0

K-Y Jelly has a perfect pH and an osmolality five to six times the ideal. Astroglide Liquid is the highest reading in the literature. Both are still the two best selling lubes in the country.

The newer brands built their whole pitch on this number, and it shows. Good Clean Love and Sliquid were the two products in the Dezzutti study that caused no measurable damage to tissue. Since then, FDA 510(k) filings have made it possible to check dozens more, which is what the lube index does.

Does it matter for everyone?

Honestly, less for some people than others. If you use a little lube occasionally and have never had a problem, a hypertonic lube is probably costing you nothing you notice. It matters a lot more if you:

• have menopause related dryness or thinning tissue (atrophy), where the epithelium is already fragile
• get recurrent yeast infections, bacterial vaginosis or urinary tract infections
• use lube for anal sex, where the WHO notes the effect of high osmolality may be greater
• are trying to conceive (sperm are sensitive to it; fertility lubes are formulated separately)
• use a lot of lube, often

For those groups the switch to an iso-osmolar product is the single cheapest change available.

Silicone and oil: the number does not apply.

Osmolality is a property of water solutions. Silicone lubes (dimethicone and friends) and oil based lubes contain no water, so there is nothing to measure. They cannot dehydrate tissue by osmosis. Their trade offs are different: silicone should not be used on silicone toys, and oils destroy latex and polyisoprene condoms. For many people with significant dryness, a silicone lube is the most comfortable option precisely because it sidesteps this whole chapter.

How to use this.

Look the product up. The index lists every published value with its source. If your lube is not there, search the FDA 510(k) database for the brand name; the summary usually gives osmolality and pH ranges. If the brand publishes nothing, assume it is high, because the ones with good numbers tend to say so.