Noscapine, also known as narcotine, nectodon, nospen, anarcotine and (archaic) opiane, is a benzylisoquinoline alkaloid of the phthalideisoquinoline structural subgroup, which has been isolated from numerous species of the family Papaveraceae (poppy family). It lacks effects associated with opioids such as sedation, euphoria, or analgesia (pain-relief) and lacks addictive potential. Noscapine is primarily used for its antitussive (cough-suppressing) effects.

Noscapine is often used as an antitussive medication. A 2012 Dutch guideline, however, does not recommend its use for acute coughing.

Noscapine can increase the effects of centrally sedating substances such as alcohol and hypnotics.

The drug should not be taken with monoamine oxidase inhibitors (MAOIs), as unknown and potentially fatal effects may occur.^{[citation needed]}

Noscapine should not be taken in conjunction with warfarin as the anticoagulant effects of warfarin may be increased.

The biosynthesis of noscapine in P. somniferum begins with chorismic acid, which is synthesized via the shikimate pathway from erythrose 4-phosphate and phosphoenolpyruvate. Chorismic acid is a precursor to the amino acid tyrosine, the source of nitrogen in benzylisoquinoline alkaloids. Tyrosine can undergo a PLP-mediated transamination to form 4-hydroxyphenylpyruvic acid (4-HPP), followed by a TPP-mediated decarboxylation to form 4-hydroxyphenylacetaldehyde (4-HPAA). Tyrosine can also be hydroxylated to form 3,4-dihydroxyphenylalanine (DOPA), followed by a PLP-mediated decarboxylation to form dopamine. Norcoclaurine synthase (NCS) catalyzes a Pictet-Spengler reaction between 4-HPAA and dopamine to synthesize (S)-norcoclaurine, providing the characteristic benzylisoquinoline scaffold. (S)-Norcoclaurine is sequentially 6-O-methylated (6OMT), N-methylated (CNMT), 3-hydroxylated (NMCH), and 4′-O-methylated (4′OMT), with the use of cofactors S-adenosyl-methionine (SAM) and NADP⁺ for methylations and hydroxylations, respectively. These reactions produce (S)-reticuline, a key branchpoint intermediate in the biosynthesis of benzylisoquinoline alkaloids.

The remainder of the noscapine biosynthetic pathway is largely governed by a single biosynthetic 10-gene cluster. Genes comprising the cluster encode enzymes responsible for nine of the eleven remaining chemical transformations. First, berberine bridge enzyme (BBE), an enzyme not encoded by the cluster, forms the fused four-ring structure in (S)-scoulerine. BBE uses O₂ as an oxidant and is aided by cofactor flavin adenine dinucleotide (FAD). Next, an O-methyltransferase (SOMT) methylates the 9-hydroxyl group. Canadine synthase (CAS) catalyzes the formation of a unique C2-C3 methylenedioxy bridge in (S)-canadine. An N-methylation (TNMT) and two hydroxylations (CYP82Y1, CYP82X2) follow, aided by SAM and O₂/NADPH, respectively. The C13 alcohol is then acetylated by an acetyltransferase (AT1) using acetyl-CoA. Another cytochrome P450 enzyme (CYP82X1) catalyzes the hydroxylation of C8, and the newly formed hemiaminal spontaneously cleaves, yielding a tertiary amine and aldehyde. A methyltransferase heterodimer (OMT2:OMT3) catalyzes a SAM-mediated O-methylation on C4′. The O-acetyl group is then cleaved by a carboxylesterase (CXE1), yielding an alcohol which immediately reacts with the neighboring C1 aldehyde to form a hemiacetal in a new five-membered ring. The apparent counteractivity between AT1 and CXE1 suggests that acetylation in this context is employed as a protective group, preventing hemiacetal formation until the ester is enzymatically cleaved. Finally, an NAD⁺-dependent short-chain dehydrogenase (NOS) oxidizes the hemiacetal to a lactone, completing noscapine biosynthesis.

Noscapine's antitussive effects appear to be primarily mediated by its σ–receptor agonist activity. Evidence for this mechanism is suggested by experimental evidence in rats. Pretreatment with rimcazole, a σ-specific antagonist, causes a dose-dependent reduction in antitussive activity of noscapine. Noscapine, and its synthetic derivatives called noscapinoids, are known to interact with microtubules and inhibit cancer cell proliferation