Osmolality

Osmolality measures how concentrated a lubricant is — specifically, how many dissolved particles it contains compared to your body's own fluids. For anal play, it matters because the rectal lining is thin and absorbs things quickly. A lube that is much more concentrated than your body (high osmolality) can pull moisture out of the tissue, which can cause irritation and make small tears more likely. The World Health Organization recommends lubricants stay below 380 mOsm/kg for this reason.

Anal tissue does not produce its own lubrication, so what you use on it matters more than it does elsewhere. A lube with high osmolality can feel slippery at first but leave the tissue dry and sore afterward — and you might not notice until later, because the rectum has fewer nerve endings than other areas. Low-osmolality lubes (under 380 mOsm/kg) stay closer to your body's natural balance, which means less chance of irritation and a more comfortable experience overall.

Most water-based lubes sit well above that threshold — some as high as 4,000 mOsm/kg — because glycerin and propylene glycol, the ingredients that make them slippery, also drive osmolality up. Silicone and oil-based lubes do not have osmolality in the same way, because they do not dissolve in water. If osmolality is a concern for you, those are the categories to look at first.

We cover this in more detail, with specific product examples and WHO guidance, in Lubricant osmolality and pH: what the numbers mean.

Lubricant osmolality and pH: what the numbers mean

The Snug Report is published by the team behind Coy, a New Zealand retailer that stocks many of the products reviewed here. We say so on every page. Our scores come from aggregated public reviews, not from us — the method is on the methodology page and the evidence count is on every review.