Amino acid blends or complexes are included in some hair loss or hair thinning treatments, primarily based on the biological role of amino acids as fundamental building blocks of keratin, the main structural protein in hair. Some scientific rationale exists: sufficient dietary protein and amino acids—such as cysteine, methionine, lysine, and arginine—are necessary for healthy hair growth, as deficiencies can impair hair structure and cycling. Small clinical studies have explored supplementation with specific amino acids (especially cystine and methionine), sometimes in combination with other nutrients like B vitamins or minerals, showing modest improvements in hair density or reduced shedding in some cases. However, these studies are often limited by small sample sizes, short durations, and use of multinutrient formulations rather than isolated amino acids, making it difficult to attribute effects specifically to amino acids.

There is a lack of large, high-quality randomized controlled trials that confirm the effectiveness of generic "amino acid blends" for hair loss in otherwise healthy individuals. However, supplementation may be justified in cases of dietary protein deficiency or malnutrition. Overall, while biological plausibility and some supportive data exist, the strength of clinical evidence for generalized amino acid complex supplementation in hair loss or thinning is modest.