butabarbital will reduce the level or outcome of nifedipine by affecting hepatic/intestinal enzyme CYP3A4 metabolism. Slight/Importance Unknown.butabarbital will lower the level or outcome of fosphenytoin by influencing hepatic enzyme CYP2C9/10 metabolism. Use Caution/Keep track of.Teniposide: Barbiturates may well decrease the serum concentration