House Wiki

Adenosine is a naturally occurring chemical in the body which in its pharmaceutical preparation is used in the treatment of tachycardia and arythmia.

Adenosine has several functions in the human body.  For example, energy use in the body is governed by adenosine phosphate molecules.  It can also prevent damage to tissue due to lack of oxygen and seizure.  It also has anti-inflammation properties and a topical preparation can be used to treat external tissue damage as it promotes healing.  It has a general effect of inhibiting nerve function.  However, in emergency medicine, it's main benefit is to cause the relaxation of the smooth muscles, particularly the heart.  As such, in cases of increased heart rate or arythmia, intravenous administration of adenosine will have the effect of slowing and regularizing the heart beat.  However, because adenosine is almost immediately processed by the body's tissues, any injection must be very near the heart to have the desired effect, and to have long term effect, it must be added to an intravenous line. 

Adenosine does not work well with persons who have consumed large amounts of caffeine as both caffeine and adenosine match up with the same molecular receptors, caffeine remains in the body longer, and the receptors are usually more receptive to caffeine.  Adenosine should also be avoided when toxins or other drugs have caused the tachycardia, the patient has athsma, when the patient has severe heart blockage, where the atrium is fluttering or in fibrillation, or if the patient has sick sinus syndrome.

Other side effects are minor - flushing of the face, lightheadedness, nausea, a metallic taste in the mouth and pain immediately after injection are common and manageable.

Adenosine at Wikipedia