Capsaicin is the molecule behind pepper heat, and it has a formula: C₁₈H₂₇NO₃. It belongs to a family of related alkaloids called capsaicinoids, and capsaicin plus dihydrocapsaicin account for roughly 90 percent of the total in a typical pod. The plant builds all of them in the placenta, the pale ribbed tissue that holds the seeds against the pod wall, and nowhere else.
The capsaicinoid family
Five compounds do nearly all the work. A representative analysis of chile extract splits out as capsaicin near 69 percent, dihydrocapsaicin 22 percent, nordihydrocapsaicin 7 percent, and homocapsaicin and homodihydrocapsaicin around 1 percent each. Each one carries the same three parts: a vanillyl ring, an amide bond, and a fatty acid tail. The tail is the variable. Its length and saturation change how fast the burn arrives and how long it stays. Dihydrocapsaicin differs from capsaicin by two hydrogen atoms, C₁₈H₂₉NO₃, and nothing else.
How the plant builds them
Two pathways converge in the placental tissue. The phenylpropanoid pathway supplies vanillylamine. The branched-chain amino acids valine and leucine supply the fatty acid. An acyltransferase joins the two, and that enzyme comes from the Pun1 locus, also written AT3 and often called capsaicin synthase. Stewart and colleagues mapped Pun1 to chromosome 2 in 2005. Sweet peppers carry two copies of the broken allele, pun1/pun1, so the plant assembles every precursor and then stalls at the final step.
Are the seeds the hottest part of a pepper?
No. The placenta holds the capsaicinoid glands. Seeds only pick up heat by touching that tissue. Scrape out the white ribs and most of the burn leaves with them; pick out the seeds and leave the ribs and you have changed almost nothing. In the hottest Capsicum chinense lines, placental tissue makes up a large share of pod weight, which is part of why they test where they do.
What capsaicin does to a nerve
Capsaicin binds TRPV1, a cation channel on pain-sensing neurons. Binding drops the channel's activation threshold, calcium floods the cell, the neuron fires, and the brain reads that signal as heat. Repeat the exposure and the nerve ending defunctionalizes. That blunting is not a side effect of the spice habit, it is the entire pharmacological basis for capsaicin as a drug.
Does capsaicin have proven medical uses?
Yes, in one narrow and well documented form. The FDA cleared Qutenza, a topical patch carrying 8 percent capsaicin and 179 mg of drug per patch, for postherpetic neuralgia in 2009 and for diabetic peripheral neuropathy of the feet in 2020. A clinician applies it in a single 30 or 60 minute session, no more often than once every three months. Claims about thermogenesis, appetite suppression and tumor cells circulate constantly; none of them has cleared that bar, so read them as open research rather than settled fact.
Why birds eat superhots and mammals will not
Birds carry a TRPV1 that capsaicin does not open. Jordt and Julius traced the difference to two residues, Tyr511 and Thr550, by swapping domains between rat and chicken receptors (Cell, 2002). The ecology follows from the chemistry. Tewksbury and Nabhan showed that pungency in wild chiles turns away rodents, which destroy the seeds they eat, while birds pass them intact (Nature, 2001). Later work from the same group tied capsaicinoids to resistance against Fusarium, the fungus that kills wild chile seeds before dispersal.
Heat by cultivar
These ranges come from the WikiPepper database, not from single record pods. Read the peer-reviewed figures in the right column as measurements of one trial, not as a ceiling.
| Pepper | Species | Database range (SHU) | Published lab figure |
| Carolina Reaper | C. chinense | 500,000 to 1,600,000 | 1,641,183 average (Guinness, 2017) |
| Trinidad Moruga Scorpion | C. chinense | 500,000 to 1,400,000 | none published by a record body |
| Ghost Pepper | C. chinense | 855,000 to 1,041,427 | 1,001,304 (Bosland and Baral, 2007) |
| Red Savina Habanero | C. chinense | 350,000 to 577,000 | 248,556 in the 2005 NMSU trial |
| Tabasco | C. frutescens | 30,000 to 50,000 | none |
| Jalapeño | C. annuum | 2,500 to 8,000 | none |
| Bell pepper | C. annuum | 0 | pun1/pun1, no capsaicinoids at all |
The Red Savina line makes the point better than any argument. Bosland and Baral grew the old record holder next to Bhut Jolokia at Las Cruces in 2005 and measured it at 248,556 SHU, far under its 577,000 record. Growing conditions moved it by more than half. Every published range hides that much variation, which is why the Universal Pepper Number assigns a single 0 to 9 heat digit from the top of the range instead of pretending to precision. The superhot list collects the varieties that clear 1,000,000 SHU at the top end.
Capsinoids, the analogs that do not burn
The Japanese cultivar CH-19 Sweet produces capsinoids instead of capsaicinoids. Capsiate, the main one, swaps the amide bond for an ester. It still activates TRPV1 in cultured cells, but it provokes almost no response in the mouth, because it hydrolyzes into vanillyl alcohol and a fatty acid before it reaches the nerve endings under the epithelium. A functional loss in the pAMT gene causes the switch.
Risks worth respecting
Large oral doses of concentrated extract irritate the gut and cause vomiting. Repeated topical overuse damages sensory nerve endings, which is the same mechanism the patch exploits on purpose. Capsaicin dissolves in oil and alcohol, not water, so after handling superhots wash with oil first and then soap. For what actually calms a burn, see the capsaicin relief guide; for how these compounds turn into a number, see the Scoville system guide.
Sources & Further Reading
- Bosland, P.W., and J.B. Baral. 'Bhut Jolokia', the World's Hottest Known Chile Pepper, is a Putative Naturally Occurring Interspecific Hybrid. HortScience 42(2):222-224, 2007.
- Stewart, C., et al. The Pun1 gene for pungency in pepper encodes a putative acyltransferase. The Plant Journal 42(5):675-688, 2005.
- Jordt, S.E., and D. Julius. Molecular Basis for Species-Specific Sensitivity to "Hot" Chili Peppers. Cell 108(3):421-430, 2002.
- Tewksbury, J.J., and G.P. Nabhan. Seed dispersal: Directed deterrence by capsaicin in chillies. Nature 412:403-404, 2001.
- U.S. Food and Drug Administration. QUTENZA (capsaicin) topical system, prescribing information, 2020.
- Iida, T., et al. TRPV1 activation and induction of nociceptive response by a non-pungent capsaicin-like compound, capsiate. Neuropharmacology 44(7):958-967, 2003.
- Priest, C.T., and D.J. Austin. The Chile Pepper Almanac. Harambe Publishing, 2026. Amazon