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noun

the chemically active part of a chemical compound
Contemporary Examples

I take a triptan called Treximet, which also includes a dose of the painkiller naproxen, the active ingredient in Aleve.
Presidents Can Have Migraines Dana Goldstein July 19, 2011

The National Institute on Drug Abuse reports the prevalence of MDMA (the active ingredient in Molly) is 12.9% in youth age 18-25.
Miley, Molly, and Your Teen’s Mind Dr. Anand Veeravagu, MD, Tej Azad May 12, 2014

In 2008, the drug heparin was tied to dozens of deaths after most of the active ingredient was swapped with a counterfeit.
Museum of Fakes for a City of Fakes William O’Connor June 24, 2014

The active ingredient is Benzyl Nicotinate (niacinamide), part of the drug group of vasodilators that widen blood vessels.
My Great Fake Bake Experiment Daniel Nester January 5, 2009

The active ingredient in the spray tan is dihydroxyacetone (DHA), the same as in over-the-counter bronzers.
My Great Fake Bake Experiment Daniel Nester January 5, 2009

Historical Examples

We are told that the active ingredient in this compound is Arnica montana, with a basis of soap cerate.
Cooley’s Practical Receipts, Volume II Arnold Cooley

This is sold in tubes or in tablets, and it is the active ingredient of the peptogenic milk powder.
The Care and Feeding of Children L. Emmett Holt

The enzyme, pancreatin, is the active ingredient, and may be prepared from the intestines of the pig.
Leather K. J. Adcock

Strychnine is a powerful poison, and forms the active ingredient of many ‘vermin-killers.’
Aids to Forensic Medicine and Toxicology W. G. Aitchison Robertson

The oeuf coquatri was supposed to be the product of a very old cock and to furnish the most active ingredient of witch ointment.
The Criminal Prosecution and Capital Punishment of Animals Edmund P. Evans