Chlorella can cause constipation, particularly during the first two weeks of use. In a clinical trial published in the World Journal of Gastroenterology, constipation was one of the two main side effects reported, alongside diarrhea. People with a history of constipation are especially likely to notice it: about 31% of participants who already dealt with constipation said their symptoms got worse during the first two weeks of chlorella supplementation.
How Common Is Constipation From Chlorella?
Constipation from chlorella is a recognized side effect, though it’s not universal. In the clinical trial, one participant dropped out entirely because constipation hit within the first two days of taking the supplement. That case resolved as soon as the person stopped taking chlorella, which confirms the supplement was the cause rather than something coincidental.
The pattern in the research suggests constipation is most likely to affect people who are already prone to it. If you’ve never had issues with regularity, your risk is lower. But if constipation is something you deal with periodically, chlorella may make things noticeably worse before your body adjusts. The good news: for most people in the trial, digestive side effects (both constipation and diarrhea) improved after the first two weeks.
Why Chlorella Affects Your Digestion
The answer comes down to fiber. More than 65% of the carbohydrate content in chlorella is dietary fiber, most of it derived from the algae’s tough cellulose cell wall. That’s a significant amount of fiber packed into a small supplement. Research confirms that the fiber in chlorella increases stool volume in humans, which can go either way: it may promote regularity for some people, or it may slow things down for others whose systems aren’t used to that fiber load.
This is similar to what happens when you suddenly increase fiber from any source. Adding a burst of fiber without enough water or without gradually building up the dose can bulk up stool faster than your gut moves it along, leading to that backed-up feeling. The fiber itself isn’t harmful, but the sudden introduction of it is what tends to cause problems.
Cell Wall Processing Matters
Humans can’t digest chlorella cells in their natural state because the cell walls are made of cellulose, the same tough material found in plant cell walls. Most chlorella supplements are mechanically processed to crack or break open the cell wall, which makes the protein and nutrients inside accessible. With broken cell walls, more than 80% of chlorella’s protein becomes digestible.
However, even in broken-cell-wall products, the cellulose fragments still contribute indigestible fiber to your diet. A “cracked” or “broken cell wall” label on your chlorella supplement means you’ll absorb more nutrients, but it doesn’t eliminate the fiber content that can contribute to constipation. If you’re comparing products, broken cell wall chlorella is generally easier on digestion overall, but it won’t completely remove the risk of digestive side effects.
Dosage and the Adjustment Window
Most clinical studies use between 6 and 10 grams of chlorella per day, with doses up to 10 to 15 grams appearing safe. Cardiovascular benefits have been observed at doses as low as 4 grams daily. There’s no established “optimal” dose, but higher amounts naturally deliver more fiber and increase the chance of digestive side effects.
The first two weeks appear to be the critical adjustment window. That’s when constipation and diarrhea are most commonly reported in trials. After that period, most people’s digestive systems seem to adapt. If you’re starting chlorella for the first time, beginning with a lower dose (2 to 3 grams per day) and increasing gradually over a week or two gives your gut time to adjust to the added fiber. Drinking extra water during this period also helps, since fiber absorbs water in the digestive tract. Without adequate hydration, high-fiber intake is more likely to harden stool rather than move it along.
Constipation vs. Diarrhea: Which Is More Likely?
Both constipation and diarrhea show up in clinical reports, and which one you experience likely depends on your baseline digestion. In the hepatitis C trial, about 31% of constipation-prone participants reported worsening symptoms, while roughly 15% of participants experienced mild diarrhea during the first week. The diarrhea cases resolved faster, improving after week two.
Your personal history is the best predictor. If you tend toward loose stools, chlorella’s fiber may actually firm things up in a helpful way. If you tend toward constipation, the sudden fiber increase is more likely to make that worse before it gets better. Neither side effect is dangerous in most cases, and both tend to be temporary. If constipation persists beyond two to three weeks or becomes severe, stopping the supplement is the simplest test: in the clinical trial, symptoms resolved once chlorella was discontinued.