If you've ever tried a natural deodorant and ended up with a rash, redness or an itchy burning feeling under your arms, you're not imagining it, and you're not alone. It's one of the most common reasons people give up on natural deodorant altogether and go back to conventional antiperspirant — not because natural doesn't work, but because one specific ingredient didn't work for their skin.
That ingredient is almost always bicarbonate of soda, also known as baking soda.
Why bicarb is in so many natural deodorants
Bicarbonate of soda is cheap, widely available and genuinely effective at neutralising odour, which is exactly why it became the go-to ingredient when the natural deodorant category took off. It works by creating an alkaline environment that odour-causing bacteria don't like. For a lot of people, that's the end of the story — it does its job with no issues at all.
But underarm skin naturally sits at a fairly neutral to slightly acidic pH, and bicarb is highly alkaline. For a portion of the population — and it's a genuinely large portion, based on how often this comes up — that mismatch irritates the skin barrier. The result is redness, itching, tightness or a rash that can appear within days of switching, especially with daily use in a warm, enclosed area like the underarm.
This is also why the reaction can feel so confusing. People often assume they're allergic to "natural" deodorant as a category, when really it's a reaction to one specific, very common ingredient.
Why Si:zen doesn't use it
We built Si:zen specifically for sensitive skin, which meant bicarb was off the table from the start. Instead, our formula uses magnesium hydroxide to neutralise odour-causing bacteria. It's doing a similar job to bicarb — targeting the source of the odour rather than masking it — but it's much gentler on skin and doesn't create the same pH clash.
We tested our formula with a focus group that specifically included women who'd had bad experiences with bicarb-based natural deodorants before, to make sure it held up for the sensitive skin cases, not just the easy ones. That was a deliberate choice, not an accident — sensitive skin was the whole starting point for Si:zen.
To be fair to bicarb
None of this means bicarb-based deodorants are bad products. If one has always worked for you with no irritation, there's genuinely no reason to switch. This isn't about one ingredient being universally wrong — it's about knowing what's most likely behind a reaction if you're currently dealing with one, so you're not writing off the entire natural deodorant category because of a single ingredient.
What to do if you think bicarb is the problem
Check the ingredients list on whatever you're currently using. If sodium bicarbonate (or bicarbonate of soda) appears near the top, it's the most likely cause of any irritation you're experiencing. From there, look for a magnesium-based, bicarb-free alternative, and as with any new product on sensitive skin, patch test on a small area first before applying it daily.
You can see Si:zen's full ingredient list on our product page, and more on how we approach sensitive skin in our FAQs.