A patient is taking 300 mg theophylline (F=1) every 8 hr. Calculate the average s.s. conc (CLΤ=2.4L/hr, Vd=30L). If this patient started taking cimetidine which is known to inhibit theophylline elimination by 50% (new CLΤ=1.2 L/hr), what will be the new average s.s. conc?
CLΤ (L/hr)
The decrease in theophylline
CLΤ leads to proportional
increase in the steady state
theophylline concentration.
Cpaverage ss=
(1) 300 mg
2.4L/hr x 8hr
= 15.6 mg/L
Cpaverage ss=
(1) 300 mg
1.2L/hr x 8hr
= 31.2 mg/L
The steady state conc increased by 100%. The new steady state conc is much higher than theophyllie therapeutic range and can cause serious toxicity. This is a serious drug interaction.
