Amoxicillin

C 16
H 19
N 3
O 5
S 1

Ball-and-stick Stick Space-filling

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44 atoms, 46 bonds.

IUPAC name: (2S,5R,6R)-6-[[(2R)-2-amino-2-(4-hydroxyphenyl)acetyl]amino]-3,3-dimethyl-7-oxo-4-thia-1-azabicyclo[3.2.0]heptane-2-carboxylic acid
Molar mass: 365.404(64) g/mol
Atoms: 44
SMILES: CC1(C(N2C(S1)C(C2=O)NC(=O)C(C3=CC=C(C=C3)O)N)C(=O)O)C
InChIKey: LSQZJLSUYDQPKJ-NJBDSQKTSA-N
PubChem CID: 33613

Made of

C H N O S

Structural formula of amoxicillin. Each unlabelled corner is a carbon atom, and the eight hydrogens on carbon are not drawn. Wedges come toward you, hashes go back. It shows which atoms are joined to which, not the shape. The model above shows the shape.

Among the penicillins, amoxicillin is defined by its side chain. ChEBI (Chemical Entities of Biological Interest) identifies it as the penicillin whose penam ring, the fused pair of rings at its core, bears a 2-amino-2-(4-hydroxyphenyl)acetamido side chain at position 6. The National Cancer Institute’s thesaurus calls it a broad-spectrum, semisynthetic aminopenicillin, active against a wide range of bacteria. It is made from 6-aminopenicillanic acid and D-(−)-2-(p-hydroxyphenyl)glycine, according to HSDB, the Hazardous Substances Data Bank.

In PubChem’s IUPAC name, bicyclo[3.2.0] marks the fused rings, 4-thia the sulfur in one of them and 7-oxo the C=O on the other. The World Health Organization (WHO) classes it among the extended-spectrum beta-lactam penicillins, a family whose ring and action are set out on the penicillin G page. MeSH, the US National Library of Medicine’s subject headings, finds it similar to ampicillin except in its resistance to stomach acid.

Structure and bonding

Amoxicillin’s side chain starts at a six-carbon ring with an O–H on its far corner, then passes a carbon bearing an NH₂ group. An amide link, C=O beside N–H, ties it to two fused rings. The four-membered ring holds a nitrogen and three carbon atoms, one of them in a C=O. Sharing its N–C edge, a five-membered ring carries the sulfur, two methyl (CH₃) groups and a carboxylic acid group, COOH.

Space-filling shows the computed molecule stretched out, the fused rings at one end and the O–H ring at the other. In Stick, the six-carbon ring has three double bonds set between three single ones, although the model’s six ring bonds all measure 1.383 to 1.395 Å. In benzene, as Chem1 describes it, those second bonds are spread round the ring. The model is also neutral, while ChEBI names it the conjugate acid of amoxicillin(1−).

Where it turns up

In the United States amoxicillin is a prescription medicine, approved by the Food and Drug Administration (FDA) against bacterial infections. Its approved list covers the ear, nose and throat, the skin, the urinary and genital tract and the airways, pneumonia included. WHO places it on its Model List of Essential Medicines, in the Access group of antibiotics. Its US brand names in the National Cancer Institute’s drug dictionary include Amoxil, Polymox, Trimox, Utimox and Wymox.

It is an animal drug too, listed in FDA’s Green Book, with uses in cats and in dogs, the canine ones including tonsillitis, tracheobronchitis and cystitis, according to HSDB. HSDB records a tolerance, a legal ceiling, for negligible residues of the drug in milk and in the uncooked edible tissue of cattle. The NORMAN Suspect List Exchange puts national use at 1,120,000 kg in the United States in 2002 and 141,761 kg in Germany in 2009.

Why it matters

Antimicrobial resistance, in the words of the US Centers for Disease Control and Prevention (CDC), is germs gaining the means to defeat drugs meant to kill them. It arises naturally, but antibiotics speed it up by pressing bacteria to adapt: the resistant germs survive, multiply and spread. Among the defences CDC describes are keeping a drug out, pumping it out, changing its target, bypassing that target, and changing or destroying the drug with enzymes. Carbapenemases are one such kind, CDC notes, and they break down carbapenems and most other beta-lactam drugs, the class amoxicillin belongs to.

Amoxicillin capsules are labelled, for infections caused by Escherichia coli, Proteus mirabilis or Enterococcus faecalis, only against beta-lactamase-negative strains, HSDB records. Augmentin XR pairs amoxicillin with clavulanate, which FDA calls a beta-lactamase inhibitor. FDA has logged seven shortage reports on amoxicillin and two on Augmentin XR, and traces rising antibiotic shortages mainly to fragile global supply chains. In December 2025 Augmentin XR became the first product approved through FDA’s Commissioner’s National Priority Voucher pilot, an approval the agency expects to help ease them.

Properties

Properties of amoxicillin Molar mass summed from IUPAC/CIAAW 2024 standard atomic weights; formula, atom and bond counts and PubChem's molecular weight from PubChem CID 33613.
Property	Value	Unit	Source
Molecular formula (Hill order)	C₁₆H₁₉N₃O₅S
Molar mass	365.404(64) g/mol	g/mol	IUPAC/CIAAW 2024
Carbon atoms	16
Hydrogen atoms	19
Nitrogen atoms	3
Oxygen atoms	5
Sulfur atoms	1
Bonds	46
Single bonds	40
Double bonds	6
PubChem molecular weight	365.4 g/mol	g/mol	PubChem

Safety

Amoxicillin brings on a red skin rash in a high proportion of patients with mononucleosis, HSDB records. All 59 company reports behind its GHS (Globally Harmonized System) classification call it a skin sensitiser, able to cause an allergic skin reaction. In 98.3 per cent, breathing it in may also bring on allergy or asthma symptoms. Haz-Map, an occupational health database, records one confirmed case of occupational asthma in a pharmaceutical worker, and the PubChem Laboratory Chemical Safety Summary holds the full classification.

Questions

Does amoxicillin contain sulfur?

Yes, a single atom of it. The sulfur sits in the five-membered ring at the molecule's core, bonded to two carbon atoms, and that ring shares an edge with a four-membered ring holding the nitrogen and a C=O. In PubChem's computed 3D model the angle at the sulfur is 94.3°, close to a right angle, and its two C–S bonds measure 1.833 and 1.861 Å. It is the only sulfur atom among the molecule's 44 atoms, hydrogens included.

Is amoxicillin trihydrate the same as amoxicillin?

Not exactly. Many amoxicillin medicines are made with the trihydrate, HSDB records, and Augmentin XR also holds amoxicillin sodium. PubChem's record, which the 3D model draws, is the anhydrous or water-free parent, and FDA classes amoxicillin anhydrous as a penicillin-class antibacterial. The trihydrate itself is an off-white crystalline powder that does not dissolve in hexane, chloroform or ethyl acetate, according to HSDB.

Can amoxicillin treat a cold or the flu?

No. Colds and flu are among the viral illnesses that antibiotics and antifungals cannot treat, CDC states. ChEBI gives amoxicillin the role of an antibacterial drug. Its approved uses in the United States are infections caused by bacteria, in places such as the ear, nose and throat, the skin and the urinary tract.

When was amoxicillin invented?

Its preparation was patented by Nayler and Smith in the United Kingdom in 1964 and in the United States in 1965, HSDB records. The drug database ChEMBL dates its first approval as a medicine to 1974 and lists it as available on prescription only.

Made of the same elements

C₁₆H₁₈N₂O₄S Penicillin G 41 atoms, 43 bonds.
C₃H₇NO₂S Cysteine 14 atoms, 13 bonds.