Can You Mix the Last Dregs of Liquid Soap with Water to Make it Last Longer?

We all add water to soap or shampoo to make those final drops last longer. But should we?

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It starts as an emergency move: You’re trying to wash your hands and the soap dispenser is giving you nothing but air, so to help that last half inch of soap reach the frustratingly short plastic tube, you add some water, give it a shake, and hey presto! Back in slightly-more-watery action. For many of us, though, this so-called emergency measure ends up being used for at least another week before we get around to replacing it. Which raises the question, does this increasingly watered-down mixture actually do the job required of it?

According to Fadi Mourad, DSC’s resident product wiz, the answer is an emphatic no. “You should never dilute a product—whether it’s soap, shampoo, conditioner, face wash, etc.—for two main reasons,” he tells us. “First off, you decrease the efficacy of the formula. The ingredients within that formula are very finely balanced: Think of it like baking a cake—you change that balance, you get a product that doesn’t work or behave like it’s supposed to.”

In other words, a shampoo specifically designed to, say, deal with dry hair will no longer be as effective at doing the thing you bought it to do. But when it comes to soap, Mourad’s second reason is far more disturbing.

“This second reason deals with safety,” he says. “Every formula is carefully preserved to protect the user until the product is done. By adding more water to the formula, you’re diluting the strength of those ingredients and adding the biggest culprit of bacterial growth—water—to a formula.”

Put more simply, the soap in your dispenser is specially designed to prevent bacteria from multiplying in the water that collects in there. When you dilute those preservatives with even more water, the very substance you’re using to remove the germs from your hands becomes itself a small germ factory—by using it, you’re actually making your hands dirtier than they were in the first place.

As the soapmaker in this Reddit thread points out, it’s easy to ignore this since germs are too small to see with the naked eye, and most of the time, we don’t leave this watered-down soap around long enough to see mold or fungus begin to grow. But if you want to see the end result, they shared this image of what happens to lotion after six months when it’s not preserved correctly. If you don’t want to scrub a smaller scale version of this into your hands every time you wash them, you might want to consider keeping some spare soaps under the sink.