Production facilities for ketracel white are a target for the Federation, because the Dominion needs it to keep the Jem'Hadar under control.
Was there a canon reason given as to why the Dominion doesn't simply replicate it?
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Fundamentally, it was a matter of control. Ketracel White was manufactured at specific facilities and given by the founder to the Vorta, who then issued it to the Jem'Hadar. Allowing the Vorta, or the Jem'Hadar, to have the capability to replicate the drug would reduce the ability to be able to control both societies.
The Son'a had a ketracel-white facility in an outpost on Devos II. The Federation became aware of its presence later in 2375, forcing Weyoun and Damar to divert warships from pursuit of the USS Defiant to the facility to protect it. (DS9: "Penumbra")
Lastly, it is possible that Ketracel White cannot be replicated at all, especially if it's a particularly complex chemical.
If the object desired contained a certain degree of complexity in its molecular structure, it could not be replicated