Many of these products are "off label" which means that they are not approved by the FDA for this purpose. Here are the off label products: L-Tryptophan ( L-Tryptophan ) Tryptophan is a non-essential amino acid. L-Tryptophan is often considered an amino acid because it can be synthesized from amino acids that are non-essential. L-Tryptophan can be used in products that contain other amino acids, but this is not recommended for sex hormone replacement products (SHRPs) as they can become dangerous over time and cause harm. For more information on the use of non-essential amino acids in supplements, go here. Tryptophan is often considered a useful amino acid for treating conditions such as anemia, but it can also cause severe kidney damage if used in excess. A supplement of L-Tryptophan has been shown to increase blood serotonin in studies, but there is no data in the scientific literature about how much increase this could cause in patients. A study of 100 men that supplemented 5 grams of L-Tryptophan daily for 2 months had a statistically significant increase in serum free Tryptophan compared to a placebo group. Another study, involving 10,000 men in a 12 week trial using L-Tryptophan for sexual dysfunction, saw a significant increase in sexual desire and frequency.