Carnitine is a quaternary ammonium compound involved in metabolism in most mammals, plants, and some bacteria. In support of energy metabolism, carnitine transports long-chain fatty acids from the cytosol into mitochondria to be oxidized for free energy production, and also participates in removing products of metabolism from cells. Given its key metabolic roles, carnitine is concentrated in tissues like skeletal and cardiac muscle that metabolize fatty acids as an energy source. Generally individuals, including strict vegetarians, synthesize enough L-carnitine in vivo.

Carnitine exists as one of two stereoisomers: the two enantiomers d-carnitine (S-(+)-) and l-carnitine (R-(−)-). Both are biologically active, but only l-carnitine naturally occurs in animals, and d-carnitine is toxic as it inhibits the activity of the l-form. At room temperature, pure carnitine is a whiteish powder, and a water-soluble zwitterion with relatively low toxicity. Derived from amino acids, carnitine was first extracted from meat extracts in 1905, leading to its name from Latin, "caro/carnis" or flesh.

Some individuals with genetic or medical disorders (such as preterm infants) cannot make enough carnitine, requiring dietary supplementation. Despite common carnitine supplement consumption among athletes for improved exercise performance or recovery, there is insufficient high-quality clinical evidence to indicate it provides any benefit.

The primary biological functions of carnitine in humans include the following:

Carnitine is a zwitterion, meaning it has both positive and negative charges in its structure. In an aqueous solution, L-carnitine is freely soluble and its ionizable groups, COO⁻ and N⁺(CH₃)₃, are over 90% dissociated at physiological pH (~7.4) for humans.

As an example of normal biosynthesis of carnitine in humans, a 70-kilogram (150 lb) person would produce 11–34 mg of carnitine per day. Adults eating mixed diets of red meat and other animal products ingest some 60–180 mg of carnitine per day, while vegans consume about 10–12 mg per day. Most (54–86%) carnitine obtained from the diet is absorbed in the small intestine before entering the blood. The total body content of carnitine is about 20 grams (0.71 oz) in a person weighing 70 kilograms (150 lb), with nearly all of it contained within skeletal muscle cells. Carnitine metabolizes at rates of about 400 μmol (65 mg) per day, an amount less than 1% of total body stores.

Many eukaryotes have the ability to synthesize carnitine, including humans. Humans synthesize carnitine from the substrate TML (6-N-trimethyllysine), which is in turn derived from the methylation of the amino acid lysine. TML is then hydroxylated into hydroxytrimethyllysine (HTML) by trimethyllysine dioxygenase (TMLD), requiring the presence of ascorbic acid and iron. HTML is then cleaved by HTML aldolase (HTMLA, a pyridoxal phosphate requiring enzyme), yielding 4-trimethylaminobutyraldehyde (TMABA) and glycine. TMABA is then dehydrogenated into gamma-butyrobetaine in an NAD⁺-dependent reaction, catalyzed by TMABA dehydrogenase. Gamma-butyrobetaine is then hydroxylated by gamma butyrobetaine hydroxylase (a zinc binding enzyme) into l-carnitine, requiring iron in the form of Fe²⁺.

Carnitine is involved in transporting fatty acids across the mitochondrial membrane, by forming a long chain acetylcarnitine ester and being transported by carnitine palmitoyltransferase I and carnitine palmitoyltransferase II.