Seaweed has been used in traditional medicine, particularly in East Asian cultures, for promoting hair health and treating hair loss or thinning. This traditional use is largely based on seaweed's rich content of minerals (such as iodine, zinc, and iron), vitamins (notably B vitamins and vitamin C), and polysaccharides, all thought to contribute to scalp health and hair growth. In folklore and traditional beauty regimens, seaweed extracts and seaweed-infused rinses are believed to strengthen hair, improve shine, and encourage growth, which has led to their inclusion in various shampoos and topical treatments.

However, scientific evidence directly supporting seaweed’s effectiveness for treating hair loss or thinning is limited and mostly indirect. Some laboratory studies suggest that certain compounds in seaweed (like fucoidan and alginate) may have antioxidative and anti-inflammatory properties, which could theoretically benefit scalp health. A few small studies have shown that seaweed-derived extracts may stimulate dermal papilla cells in vitro, which are important in hair follicle development, but these findings have not been robustly replicated in human clinical trials. Most clinical evidence is anecdotal or based on formulations where seaweed is one of many components.

In summary, while traditional use and the nutrient profile of seaweed provide some rationale for its inclusion in hair care, strong scientific validation for its efficacy against hair loss or thinning is lacking.