Two short-term studies found that
theophylline serum levels were slightly increased by
isoniazid and a case of
theophylline toxicity supports this finding. However,
another short-term study found that
isoniazid slightly increased
theophylline clearance.
The outcome of concurrent use is uncertain but it would be prudent to be alert for any evidence of increased
theophylline levels and
toxicity if
isoniazid is given. It may take 3 to 4 weeks for this interaction to develop.