Most commonly, these are “non-preferred” brand drugs. Interferences in immunoassays for barbiturates, benzodiazepines, bupropion, ketamine, opiate, fentanyl, methadone, marijuana, and PCP butalbital fioricet dosage for children summarized in Table 7. In 2011, however, the FDA made a change to drug formulations containing acetaminophen. Secobarbital is used as a calibrator in several commercially available immunoassays for barbiturates. This is not a complete list of side effects and others may occur. What other drugs will affect Fioricet?