Cimetidine raises
theophylline serum levels (by about one-third) and
toxicity has been seen. The interaction is unlikely to be clinically relevant in most patients with low- dose (non-prescription)
cimetidine.
Monitor
theophylline levels if
cimetidine is added or withdrawn. The peak effect appears to occur after 3 days. Alternatively, use ranitidine or
famotidine, which appear not to interact.