Sometimes it is not safe to use certain medications at the same time. Some drugs can affect your blood levels of other drugs you take, which may increase side effects or make the medications less effective.
Using guanfacine with other drugs that make you drowsy can worsen this effect. Ask your doctor before using opioid medication, a sleeping pill, a muscle relaxer, or medicine for anxiety, depression, or seizures.
Tell your doctor about all your other medicines. Some may affect guanfacine, especially:
ketoconazole;
a barbiturate, such as phenobarbital;
blood pressure medications;
medicine to treat mental illness; or
a sedative, such as Valium or Xanax.
This list is not complete. Other drugs may affect guanfacine, including prescription and over-the-counter medicines, vitamins, and herbal products. Not all possible drug interactions are listed here.
 Guanfacine drug interactions (more detail)
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