Linum usitatissimum (flaxseed, linseed) improves constipation symptoms

Linum usitatissimum (flaxseed) may help relieve constipation by increasing stool bulk and improving bowel regularity. The high fiber content (especially soluble mucilage) absorbs water, softens stools, and promotes intestinal transit.

The HMPC (from EMA) conclusions on the use of linseed for treating constipation are based on ‘well-established use’. This means that there are bibliographic data providing scientific evidence of linseed’s effectiveness and safety when used in this way, covering a period of at least 10 years in the EU.
The HMPC concluded that linseed can be used to treat habitual constipation or in conditions requiring the stool to be softer so it can pass more easily.
The HMPC also concluded that, on the basis of its long-standing use, linseed can be used to relieve mild gastrointestinal (stomach and gut) discomfort.
Note that “well-established use” is a regulatory classification, not an evidence grading system like GRADE. It reflects that the EMA considers the evidence sufficient for a medicinal indication under the EU regulatory framework.

Source: https://www.ema.europa.eu/en/medicines/herbal/lini-semen

Clinical studies indicate that Linum usitatissimum (flaxseed) can improve constipation symptoms by increasing stool frequency, improving stool consistency, and facilitating bowel movement. Its high content of dietary fiber and mucilage increases water retention in the intestinal tract, which helps soften stools and promote intestinal transit. Evidence from human trials supports its use as a fiber-based intervention for mild to moderate constipation, although effects may vary depending on dose, preparation, and individual response.

• Strong evidence

Do not consume raw or unripe flaxseeds; they can be toxic due to the presence of cyanogenic glycosides that can release (volatile) hydrocyanic acid.