Cystine is the oxidized dimer form of the amino acid cysteine. Cystine serves two biological functions, a site of redox reactions and a mechanical linkage that allows proteins to retain their 3-dimensional structure. It is common in many foods such as eggs, meat, dairy products, and whole grains as well as skin, horns and hair. Human hair and skin contain approximately 10–14% cystine by mass. Cysteine supplements are sometimes marketed as anti-aging products with claims of improved skin elasticity. Cysteine is more easily absorbed by the body than cystine, so most supplements contain cysteine rather than cystine. N-acetyl-cysteine (NAC) is better absorbed than other cysteine or cystine supplements.

Mequinol (mequinol is 4-hydroxyanisole) is an active ingredient in topical drugs used for skin depigmentation. The mechanism of action of mequinol is unknown. Although mequinol is a substrate for the enzyme tyrosinase and acts as a competitive inhibitor of the formation of melanin precursors, the clinical significance of these findings is unknown.

Amifostine is an organic thiophosphate cytoprotective agent known chemically as 2-[(3¬ aminopropyl)amino]ethanethiol dihydrogen phosphate (ester), it’s adjuvant used in cancer chemotherapy and radiotherapy involving DNA-binding chemotherapeutic agents. It is marketed under the trade name Ethyol. Amifostine is a prodrug and is dephosphorylated by alkaline phosphatase in tissues to a pharmacologically active free thiol metabolite. This metabolite is believed to be responsible for the reduction of the cumulative renal toxicity of cisplatin and for the reduction of the toxic effects of radiation on normal oral tissues. The ability of Ethyol to differentially protect normal tissues is attributed to the higher capillary alkaline phosphatase activity, higher pH and better vascularity of normal tissues relative to tumor tissue, which results in a more rapid generation of the active thiol metabolite as well as a higher rate constant for uptake into cells. The higher concentration of the thiol metabolite in normal tissues is available to bind to, and thereby detoxify, reactive metabolites of cisplatin. This thiol metabolite can also scavenge reactive oxygen species generated by exposure to either cisplatin or radiation. Healthy cells are preferentially protected because amifostine and metabolites are present in healthy cells at 100-fold greater concentrations than in tumor cells.

Cisapride is chemically related to metoclopramide, but unlike metoclopramide, it does not cross the blood-brain barrier or have antidopaminergic effects. Cisapride is a serotonin-4 (5-HT4) receptor agonist. Cisapride was indicated for the symptomatic treatment of adult patients with nocturnal heartburn due to gastroesophageal reflux disease. The Food and Drug Administration (FDA) in America stopped the marketing of cisapride as of 14th July 2000. They had received at least 341 reports of heart rhythm abnormalities and these led to 80 deaths. Other reported adverse effects are: headache, diarrhea, abdominal pain, nausea, constipation. Cisapride for animals has been found helpful in some cases of megaesophagus and is a common treatment for feline megacolon. Clarithromycin, erythromycin, and troleandomycin markedly inhibit the metabolism of cisapride. Concurrent administration of certain anticholinergic compounds, such as belladonna alkaloids and dicyclomine, would be expected to compromise the beneficial effects of cisapride.

Cetyl alcohol is the 16-carbon alcohol corresponding to palmitic acid, so called because it is isolated from among the hydrolysis products of spermaceti. This medication is used as a moisturizer to treat or prevent dry, rough, scaly, itchy skin and minor skin irritations (e.g., diaper rash, skin burns from radiation therapy). Most emollients can be used safely and effectively with no side effects. However, burning, stinging, redness, or irritation may occur. A very serious allergic reaction to this drug is rare.

Bentiromide (Chymex) is a diagnostic agent that was approved for the diagnosis of pancreatic exocrine insufficiency. It is given by mouth as a noninvasive test. The amount of p-aminobenzoic acid and its metabolites excreted in the urine is taken as a measure of the chymotrypsin-secreting activity of the pancreas. Headache and gastrointestinal disturbances have been reported in patients taking bentiromide. Bentiromide is not available in the United States or Canada.

D-Xylose is a sugar originally isolated from wood. D-Xylose is a colorless to white crystalline powder that is used in food products such as breads, butter, and beverage industry as flavoring agent and sweetener. Specifically, D-Xylose is used to sweeten products intended for use by obese and diabetic customers because it is a non-caloric sweetener. It has a cool and refreshing taste and has the same sweetness as sucrose, and it is often used to sweeten a wide variety of foods and beverages such as spices, meat products, butter, etc. D-Xylose is a sugar widely used as diabetic sweetener in food and beverage. is a sugar first isolated from wood, and named for it. Xylose is classified as a monosaccharide of the aldopentose type, which means that it contains five carbon atoms and includes a formyl functional group. It is derived from hemicellulose, one of the main constituents of biomass. Like most sugars, it can adopt several structures depending on conditions. With its free carbonyl group, it is a reducing sugar. D-xylose absorbtion test has been used for diagnosis of intestinal malabsorption.

L-threonine is an essential amino acid. Threonine is a precursor of glycine. The biochemical studies on rats proved that glycine is synthesized from threonine (through threonine dehydrogenase pathway). Threonine dehydrogenase is the key enzyme in mammals like pigs, cat, and rats for degradation of 80% threonine. In adult humans, degradation of 7–11% of threonine is done by threonine dehydrogenase. The human L-threonine 3-dehydrogenase gene (GeneID: 157739, UniProtKB: Q8IZJ6 (TDH_HUMAN)) is an expressed pseudogene having lost the splice acceptor site preceding exon 6 and codon arginine-214 (CGA) is mutated to a stop codon (TGA). A few trials demonstrated that oral L-threonine may alleviate clinical signs of amyotrophic lateral sclerosis and spasticity in humans. L-Threonine has recently been brought into agricultural industry for balancing the livestock feed.

Phenylalanine is a biologically essential amino acid that acts as a precursor to tyrosine and the catecholamines (epinephrine, norepinephrine, dopamine, and tyramine), and is a constituent of many central nervous system neuropeptides. Normal dietary levels of phenylalanine are approximately 1-2 grams daily. Phenylalanine appears in two forms which are identical mirror images of each other: L-phenylalanine, a nutritional supplement, and D-phenylalanine, an effective painkiller and antidepressant due to its ability to inhibit the breakdown of enkephalins, the brain’s natural pain killers.

Tryptophan is alpha-aminoacid that is used in the biosynthesis of proteins. It is essential aminoacid in humans, meaning the body cannot synthesize and it must be obtained from the diet. Tryptophan is a precursor of serotonin, and as such is sold over the counter in many countries as a dietary supplement for use as an antidepressant, anxiolytic and sleep aid, however application of tryptophan in these indications is not approved by FDA.