Milk of magnesia helps canker sores in two ways: it neutralizes the acids in your mouth that irritate the open wound, and it forms a protective coating over the sore that shields it from further contact with food, drinks, and bacteria. This combination reduces pain and creates a better environment for healing.
How It Works on Canker Sores
The active ingredient in milk of magnesia is magnesium hydroxide, a mild base that raises pH on contact. Your mouth is naturally slightly acidic, and that acidity spikes when you eat citrus fruits, tomatoes, vinegar-based foods, or drink coffee. For healthy tissue, this isn’t a problem. But a canker sore is an open ulcer with exposed nerve endings, and every acid contact triggers a sharp sting.
When you dab milk of magnesia directly on a canker sore, the magnesium hydroxide neutralizes acid right at the surface of the wound. This shifts the local pH toward neutral, which does two things: it immediately reduces the burning sensation, and it makes the environment less hospitable for the bacteria that can slow healing. The liquid also dries into a thin, chalky film over the ulcer. That physical barrier acts like a temporary bandage, keeping food particles and saliva from constantly aggravating the raw tissue underneath.
How to Apply It
You can use plain, unflavored milk of magnesia straight from the bottle. Shake it well first, since the magnesium hydroxide settles to the bottom. Dip a clean cotton swab into the liquid and dab it directly onto the canker sore. Hold it there for a few seconds to let it coat the area, then try not to eat or drink for at least 10 to 15 minutes so the coating has time to set.
Most people apply it three to four times a day, especially after meals and before bed. You can also use it as a mouth rinse by swishing a small amount (about a teaspoon) around your mouth for 30 seconds to a minute, then spitting it out. The rinse approach works well if you have multiple sores or sores in hard-to-reach spots like the back of your throat or the base of your gums.
Some people mix milk of magnesia with liquid diphenhydramine (the antihistamine found in liquid allergy medicine) in a 1:1 ratio. The antihistamine adds a mild numbing effect on top of the protective coating. If you try this, swish and spit rather than swallowing, since diphenhydramine causes drowsiness.
What It Can and Can’t Do
Milk of magnesia is a symptom manager, not a cure. Most canker sores heal on their own within one to two weeks regardless of treatment. What milk of magnesia does is make that week or two significantly less painful. By keeping acid and debris off the wound, it may also help your sore heal a bit faster since the tissue isn’t being constantly re-irritated.
It won’t help with cold sores, which are caused by the herpes simplex virus and appear on the outer lips. Canker sores are the ones that form inside your mouth, on the soft tissue of your cheeks, tongue, or gums. If you’re not sure which you’re dealing with: canker sores are flat, round ulcers with a white or yellowish center and a red border. Cold sores are fluid-filled blisters on or near the lip line.
Why Some People Prefer It Over Other Remedies
Compared to topical numbing gels that contain benzocaine, milk of magnesia has a few practical advantages. It’s inexpensive, widely available, and most people already have a bottle at home. It doesn’t sting on application the way salt water rinses or hydrogen peroxide can. And because it physically coats the sore rather than just numbing it, the relief tends to last longer between applications.
The downside is that the coating wears off relatively quickly, especially if you eat or drink. Medicated patches or prescription pastes adhere to the tissue more effectively and provide longer-lasting coverage. For small, occasional canker sores, milk of magnesia is a solid first-line option. For large sores (bigger than a centimeter), sores that keep coming back, or sores that last longer than two weeks, a stronger treatment is worth pursuing.
Precautions Worth Knowing
If you’re applying it topically and spitting it out, there’s very little risk for most people. The amount you might accidentally swallow during a rinse is small, but magnesium hydroxide is a laxative at higher doses, so swallowing large amounts could cause diarrhea or stomach cramps.
People with kidney disease should be more cautious. The kidneys are responsible for clearing excess magnesium from the body, and even small amounts of additional magnesium can accumulate when kidney function is impaired. Mayo Clinic lists magnesium-containing antacids among the medications to discuss with a provider if you have kidney disease. This applies even to topical oral use, since some absorption through the mucous membranes is possible.