“Magic Chloro”: Profound Effects of the Chlorine Atom in Drug Discovery

09 August 2022, Version 1
This content is a preprint and has not undergone peer review at the time of posting.


Chlorine is one of the most common atoms present in small-molecule drugs beyond carbon, hydrogen, nitrogen and oxygen. There are currently more than 250 FDA-approved chlorine-containing drugs, yet the beneficial effect of the chloro substituent has not yet been reviewed. The simple substitution of a hydrogen atom (R = H) with a chlorine atom (R = Cl) can result in incredible improvements in potency of up to 100,000-fold, and can lead to profound effects on pharmacokinetic parameters such as clearance, half-life, and drug exposure in vivo. Following the literature terminology of “magic methyl effect” in drugs, the term “magic chloro effect” has been coined herein. Reports of 500-fold or >1000-fold potency improvement are often serendipitous discoveries that can be considered “magic” rather than planned. However, hypotheses made to explain the magic chloro effect can lead to lessons that accelerate the cycle of drug discovery. With this in mind, we believe that medicinal chemists should place chlorine atoms into their lead scaffolds in judicious fashion, and organic chemists should invent more methods to place chlorine atoms selectively onto complex molecules.


Drug Discovery
Medicinal Chemistry
Magic Chloro
Potency improvement

Supplementary materials

Supplementary Information
This is a Supplementary Information file that includes tables of data that were collected from literature sources. The review was written based on these data.


Comments are not moderated before they are posted, but they can be removed by the site moderators if they are found to be in contravention of our Commenting Policy [opens in a new tab] - please read this policy before you post. Comments should be used for scholarly discussion of the content in question. You can find more information about how to use the commenting feature here [opens in a new tab] .
This site is protected by reCAPTCHA and the Google Privacy Policy [opens in a new tab] and Terms of Service [opens in a new tab] apply.