Smelly gas from probiotics: why it happens and what to do

Key facts

• "Most of the time gas does not have an odor." The odor comes from bacteria in the large intestine that release small amounts of gases containing sulfur. [MedlinePlus]
• People with flatulence may feel it has an unpleasant odor, which may be due to sulfur in the gas. [NIDDK]
• High-sulfur foods include broccoli, Brussels sprouts, cauliflower and cabbage, along with eggs, garlic, meats, nuts, onions, beer and wine. [Cleveland Clinic]
• Some probiotics produce gases as byproducts; a sudden rise can mean more bloating and gas that "should resolve within a few days." That line is about the amount of gas, not its smell. [Cleveland Clinic]
• Excessive or smelly wind can be a side effect of some medicines, among them ibuprofen and other NSAIDs, some laxatives, antifungal medicines and statins. [NHS]
• If you have been passing more gas than usual, or are concerned about the way it smells, talk to a healthcare provider. [Cleveland Clinic]

Why does gas smell? What MedlinePlus and NIDDK say

Sulfur. Most of the gas you pass has no smell; the odor comes from small amounts of sulfur-containing gases that bacteria in the large intestine give off, MedlinePlus says.

NIDDK explains where intestinal gas comes from: air you swallow, and bacteria in the large intestine breaking down sugars, starches and fiber that the stomach and small intestine did not fully digest. On the smell, it is brief:

"The odor may be due to sulfur in the flatus."

NIDDK, Symptoms and Causes of Gas in the Digestive Tract

Cleveland Clinic gastroenterologist Christine Lee, MD, names the compound: digestion sometimes produces hydrogen sulfide, which can add a rotten-egg odor. She also points out that gut chemistry differs from person to person, so some people's bacteria naturally make gas that smells a bit stronger. MedlinePlus makes the same point about amount: foods that produce gas in one person may not cause gas in another.

Do probiotics make gas smell worse?

None of the sources we checked says they do. What sources describe after starting a probiotic is more gas, for a short time, not a change in how it smells.

Cleveland Clinic writes that some probiotics produce gases in the gut as byproducts, and that a sudden rise in them can bring more bloating and gas during digestion, which should settle within a few days. That passage is about volume. It does not mention odor, and this page does not read it as covering smell.

Hard numbers are missing in general. NCCIH notes that few studies have looked at the safety of probiotics in detail, so solid information on how often side effects happen, and how severe they are, is lacking. Physician's Choice uses this summary of the NIH view: According to the NIH Office of Dietary Supplements, probiotics are generally well tolerated by healthy people, though they may cause mild digestive symptoms, such as gas. Probiotics may not be appropriate for everyone, particularly people who are seriously ill or have weakened immune systems. Talk with your healthcare provider before use.

How much extra gas is usual at the start, and when to stop, is covered in gas in the first weeks of a probiotic.

What can you do about smelly gas?

Mostly, change what and how you eat, then give it time. Cleveland Clinic says the problem usually takes care of itself when food or a medicine is behind it.

1. Look at your plate first. Dr. Lee says the symptoms should disappear once the offending item works through your system.
2. Eat smaller meals, more often. Eat and drink slowly, with your mouth closed, the NHS says.
3. Drink water, not fizz. MedlinePlus suggests lots of water and non-fizzy drinks, and eating more slowly so you swallow less air.
4. Go easy on beer, wine and fruit juice. The NHS also says not to smoke or chew gum, and to exercise regularly.
5. Keep a short food log. Cleveland Clinic says a provider may ask you to keep a journal of what you eat and drink to see whether something in your meals is causing the extra gas.
6. Ask a pharmacist. The NHS says a pharmacist can tell you whether something you can buy may help, such as simethicone, or underwear and pads that absorb smells, and whether to see a doctor.

Thinking of peppermint oil, ginger or magnesium instead? See supplements for bloating for what sources say about each.

How long does smelly gas last?

When a food or a medicine is the cause, until it has passed through, Cleveland Clinic says. No source we checked gives a time for a probiotic and smell.

Cleveland Clinic's "within a few days" for a new probiotic describes extra gas and bloating. It says nothing about odor, so it is not a timeline for smell. The signal to act is persistence: Dr. Lee advises talking to your healthcare provider if bad gas continues for an extended period, and the NHS says to see a GP if farting is affecting your life after self-help and pharmacy products have not worked.