Tyrosine

Tyrosine is one of the amino acids your body already uses as a protein building block. In skincare, it shows up as a skin-conditioning agent.

It's most notable for being the raw material that your skin's tyrosinase enzyme converts into melanin (skin pigment); this is why it's sometimes marketed as a "tan accelerator" in products.

However, the tanning claim is shaky. Lab studies on isolated skin cells show tyrosine can boost melanin production at fairly high concentrations but hasn't shown to speed up/deepen a tan when applied directly to skin in animal studies (up to 0.05%).

Finished cosmetic products use it at low levels (generally well under 1%). At these amounts, it's considered non-irritating and non-sensitizing based on repeat-use patch testing.

Allergy-wise, plain tyrosinase hasn't shown sensitization issues but a chemically modified relative called Oleoyl Tyrosine has had a handful of cases.