Tea, especially green tea, does interfere with drugs, due to catechins. Anyway, the only interactions recognized by the occidental medicine so far are two: green tea reduces the absorption of beta-blockers (especially nadolol by as much as 76%), drugs used to treat cardiovascular diseases as hypertension, myocardial infarction and heart failure. The second interaction is with bortezomib, an anticancer drug, which also is reduced in effect when given with green tea. With these drugs, tea intake is forbidden.
There could be other drug-interactions, as cathechins are known to bind to many molecules, thus blocking (or delaying) their absorption by the gastrointestinal tract, but they are still not scientifically proven. For that reason, when taking drugs other than those mentioned above, it is a good rule to let the drug be absorbed before taking tea. If you just had tea, instead, you should let the gastrointestinal tract have another meal before taking your medicine. Alternatively, a large amount of water could wash out the stomach from catechins and let the drug be absorbed without coming in contact with them.
There are also other interactions due to caffeine, but not regarding the absorption of drugs: it is due to its excitatory effect on the central nervous system, and this could interfere with those drugs which aim to depress it (as sedatives, sleep aids, etc). In this case, we see a decrease in effectiveness of these drugs, as it is easy to imagine.
