Watermelon frost (known as Xigua Shuang in Chinese) is a traditional Chinese medicine made by crystallizing watermelon rind with a mineral salt called Glauber’s salt (sodium sulfate). The white, powdery crystals that form on the outside of the rind are collected and used, most commonly as a topical treatment for mouth ulcers, sore throats, and swollen gums. Today it’s widely sold as a spray or powder, particularly under the brand name Sanjin Watermelon Frost, and is one of the most recognizable over-the-counter remedies in Chinese-speaking communities worldwide.
How Watermelon Frost Is Made
The traditional preparation method is surprisingly hands-on. A fresh watermelon is cut open and layered with Glauber’s salt inside an unglazed clay crock, alternating layers of watermelon and salt. The crock is then sealed and hung in a cool, ventilated place. Over several days, white crystals slowly form on the outside surface of the porous clay. These crystals are scraped off and collected repeatedly until no more appear. The “frost” in the name comes from the appearance of these white mineral deposits on the outside of the container, which look like a layer of frost.
This process is essentially a slow extraction. Moisture from the watermelon dissolves the salt, migrates through the unglazed clay, and evaporates on the outer surface, leaving behind a fine crystalline powder that contains both mineral compounds and water-soluble components from the watermelon itself.
What’s in Modern Watermelon Frost Products
Commercial watermelon frost sprays and powders go well beyond the simple two-ingredient original. Modern formulations typically contain 14 different traditional Chinese medicinal ingredients. Alongside the watermelon-derived crystals, you’ll find borax, menthol, and borneol (which provide cooling and mild numbing effects), plus several plant extracts with traditional reputations for reducing heat and inflammation. These include bark from the cork tree, root extracts from plants in the Sophora family, and compounds from skullcap and goldthread, both of which contain naturally bitter, antimicrobial substances.
The result is a fine powder with a cooling, slightly bitter taste that can be sprayed or puffed directly onto sore areas inside the mouth and throat.
What People Use It For
The primary use is treating canker sores (aphthous ulcers), those painful shallow sores that appear on the soft tissue inside your mouth. It’s also used for sore throats, swollen or bleeding gums, and minor throat inflammation like tonsillitis or laryngitis. In traditional Chinese medicine terms, it’s classified as a remedy that “clears heat and reduces swelling,” which roughly translates to reducing inflammation and soothing irritated mucous membranes.
A 2022 network meta-analysis published in the Journal of Healthcare Engineering reviewed studies on four types of watermelon frost combination medications for oral ulcers, suggesting there is active clinical interest in measuring its effectiveness. The menthol and borneol in the formula provide an immediate cooling and mild pain-relieving sensation on contact, which is part of what makes it popular for quick relief. Several of the herbal ingredients also have documented antimicrobial properties in laboratory settings, though large-scale clinical trials meeting Western regulatory standards are limited.
How It’s Applied
Watermelon frost comes in two main forms: a spray bottle with a thin nozzle designed to direct powder onto a specific spot, and a loose powder that can be applied with a clean finger or cotton swab. You point the nozzle at the ulcer or sore area and puff a small amount of powder directly onto it. The powder sticks to the moist tissue and dissolves gradually, creating a coating that sits on the sore. Most people apply it several times throughout the day, particularly after meals.
The sensation is immediately cooling, sometimes with a mild sting on open sores that fades quickly. Many users describe noticeable pain relief within minutes, with the powder forming a temporary protective layer over the ulcer.
Safety Concerns Worth Knowing
Watermelon frost is generally considered safe for short-term, occasional use by adults, but there are a few concerns worth understanding.
The most significant is heavy metal contamination. A case report documented a 5-year-old boy who developed mercury poisoning after using a Chinese herbal spray for recurrent mouth ulcers. The product was found to contain high mercury levels. While this doesn’t mean all watermelon frost products are contaminated, it highlights the risk of inconsistent quality control in products that aren’t regulated as pharmaceuticals in most Western countries. If you purchase watermelon frost, sourcing matters.
Borax is a standard ingredient in the formula, and boron compounds carry risks at higher exposures. Australia’s Therapeutic Goods Administration has flagged that boron overdose may have adverse consequences for fertility. Notably, the TGA pointed out that Sanjin Watermelon Frost Insufflation “does not advise additional caution for children and pregnant women,” which the agency considered a labeling concern rather than a safety endorsement. Pregnant women and young children may want to avoid it for this reason.
Because watermelon frost products are sold as traditional medicines or dietary supplements rather than approved drugs in most countries outside China, they don’t undergo the same standardized testing for purity, potency, or contaminant levels that pharmaceutical products do. The quality can vary significantly between manufacturers and even between batches from the same manufacturer.
Where to Find It
Watermelon frost is sold in most Chinese pharmacies, many Asian grocery stores, and widely available online. The Guilin Sanjin brand (often in a green box with a small spray bottle inside) is the most commonly encountered version. Prices are typically low, often just a few dollars per bottle. It remains one of the most popular home remedies for mouth sores across East and Southeast Asia, and its reputation has spread to other communities largely through word of mouth and online recommendations.