Lawyers for a Missouri inmate said Monday that they would appeal a decision not to grant a delay in his execution, arguing that the drug to be used was likely to be substandard, and thus could cause him severe pain, because it was produced in a compounding pharmacy not subject to federal drug safety regulations.

The condemned man, Herbert Smulls, 54, is scheduled to be put to death on Wednesday for the 1991 killing of a jewelry store owner. The decision to deny the stay was handed down late Monday by a federal district judge in Kansas City.

In her decision, Judge Beth Phillips ruled that simply speculating about unknown dangers associated with the drug was not enough to prove that it would cause “needless suffering.” But the judge also acknowledged that it had been impossible for Mr. Smulls to find out more information, saying it “weighs heavily on the court.”

Mr. Smulls’s lawyers say that Missouri’s Department of Corrections procured the drug, pentobarbital, from a compounding pharmacy in Oklahoma and wanted more time for the pharmacy to be investigated. They said in court filings last week that the pharmacy conducted its sterility testing at an Oklahoma laboratory that had approved medicine from a Massachusetts compounding pharmacy responsible for a national meningitis outbreak in 2012.