“"Diluting a cleanser, hand soap, or body wash with water to make it last longer disrupts the chemistry of the product's preservative system, which can render ingredients ineffective, increase irritation, and foster microbial growth." (Instagram post, @drdrayzday, published 2026-10-08. Transcript adds the instruction "Never mix water into your cleanser or your hand soap, body wash, in order to make it last longer.")”

• Where would you place this on a scale from false to accurate, and why?
• What would you need to see to be sure? Who would have the original evidence?
• Which words in the claim are doing the most work?

A widely shared post warns against topping up your cleanser, hand soap or body wash with water to make it last longer, saying this wrecks the preservative system and can lead to germs growing in the bottle. The core of that warning holds up. Preservatives are tested and approved for the product exactly as it was made, international testing standards treat available water as the key factor in whether microbes can grow, and a published laboratory study found bacteria multiplying in liquid soap that had been diluted with tap water, with more growth at higher dilutions. Public health guidance already advises against refilling or topping off liquid soap containers for the same reason. The weakest part of the post is the suggestion that diluting makes a product more irritating, which no source found here supports, and the formulation literature actually suggests lower surfactant concentration tends to be gentler, not harsher. The post also does not say how much water, and the amount matters a great deal to whether preservation fails. What remains unknown is whether people diluting cleanser at home have actually been harmed, since no case reports of infection from this specific practice were found. General information only, not medical or dermatological advice.

• A cosmetic product's preservative protection is established by testing the formulation as it is made. Under ISO 11930 the finished product is inoculated and monitored at 0 hours, 7, 14 and 28 days, so a result obtained that way does not carry over to a mixture the user has changed.
• Water availability is central to whether a formulation resists microbes. ISO 29621 uses water activity as the key measure, treating products below a water activity of 0.5 as hostile to microbial growth. Adding water moves a product in the opposite direction.
• In laboratory conditions, liquid soap diluted with tap water to 75%, 50%, 25% and 12.5% strength supported the growth of Pluralibacter gergoviae and Pseudomonas aeruginosa, with counts rising as the soap was diluted further.
• Contaminated liquid soap can leave hands with more bacteria than before washing. In a school field study, Gram-negative bacteria on hands rose by about 1.42 log10 CFU per hand after washing with soap from contaminated bulk-refillable dispensers.
• Public health guidance already discourages a close relative of this practice. CDC advises against refilling or topping off containers of liquid soap in healthcare settings because the practice has been associated with contamination.
• For any ingredient whose effect depends on its concentration, such as an antibacterial soap active or an exfoliating acid in a wash, diluting the bottle delivers less of that ingredient per use.
• The post's own wording is hedged. The transcript and caption say dilution "can" render ingredients ineffective and "can" foster microbial growth, rather than saying it always does.

• The claim that dilution will increase irritation is the weakest part and no retrieved source supports it. Where concentration is the variable, the formulation literature describes the opposite direction, with lower free surfactant concentration associated with less irritation. If diluting does raise irritation risk, the plausible routes are a contaminated product or a shifted pH, and the post does not say this.
• The post presents the harms as a single package driven by one mechanism, the preservative system. In fact three different things are being asserted with three very different levels of support: microbial growth, which is well grounded in mechanism and laboratory data; reduced ingredient activity, which follows from arithmetic for dose-dependent actives but has not been measured for home dilution; and irritation, which is unsupported.
• No amount of water is specified. The laboratory data show markedly different outcomes between a small dilution and a heavy one, so a blanket statement covers cases that are not equivalent.
• The strongest evidence available concerns liquid hand soap in dispensers, much of it from healthcare and institutional settings. Applying it to a facial cleanser in a home bathroom is reasonable on mechanism but is an extension beyond what was studied.

• Whether consumer dilution of a facial cleanser at home actually produces contamination at levels that cause skin or eye infections. No clinical report or case series of harm from this specific practice was located.
• How far a product can be diluted before its preservative system fails. This depends on the preservative used, its minimum effective concentration, the pH and the formulation, and no general threshold exists across products.
• Whether diluting measurably increases irritation, and by what route. No study testing this was found.
• Whether rinse-off products, which have short skin contact and are washed away, carry a materially different real-world risk from leave-on products once diluted. The standards treat product category as part of the risk assessment, but no comparison specific to home-diluted cleansers was found.
• The size of any loss of cleansing or active performance from dilution at ordinary household ratios.

• Lucassen R, van Leuven N, Bockmühl D. "A loophole in soap dispensers mediates contamination with Gram-negative bacteria." MicrobiologyOpen 12(5):e1384, published 2023-09-29 (primary)
  https://onlinelibrary.wiley.com/doi/full/10.1002/mbo3.1384
• CDC, "Clinical Safety: Hand Hygiene for Healthcare Workers" (guidance page on refilling and topping off soap and sanitiser dispensers) (primary)
  https://www.cdc.gov/clean-hands/hcp/clinical-safety/index.html
• ISO 11930 (Evaluation of the antimicrobial protection of a cosmetic product, 2019 + A1:2022) and ISO 29621:2017 (microbiological risk assessment, water activity thresholds) (primary)
  https://cdn.standards.iteh.ai/samples/68310/8bbc3d7f702e445bbe7716bbdc38676b/ISO-29621-2017.pdf
• Zapka CA et al., "Bacterial Hand Contamination and Transfer after Use of Contaminated Bulk-Soap-Refillable Dispensers," Applied and Environmental Microbiology, 2011 (primary)
  https://journals.asm.org/doi/full/10.1128/aem.02632-10
• FDA submission material on cosmetic microbiological safety (2012), on products being "adequately preserved against microbial contamination that may occur during consumer usage" (secondary)
  https://downloads.regulations.gov/FDA-2011-N-0770-0009/attachment_1.pdf
• Formulation patent literature describing preservative "effective amount" defined at a specified final dilution volume (tertiary)
• Surfactant formulation literature describing reduced irritation through lower free surfactant concentration (the "concentration dilution effect") (tertiary)