The two most common complications after stent plaement are in-stent stenosis and in-stent thrombosis.

Even though they rhyme, they are very different problems.

In-stent stenosis is when endothelial and smooth muscle hyperplasia cause the lumen of the stent to gradually narrow with time. This usually presents clinically as gradual onset of angina or exercise intolerence as the process occurs slowly over time. 

In-stent thrombosis is when a previously patent stent is suddenly occluded by a platelet thrombus, usually as a reaction to the bare metal of the stent coming into contact with coronary blood and activating mechanisms of platelet aggregation. This can present as acute coronary syndrome, myocardial infarction or sudden cardiac death.

The risk of in-stent thrombosis is most acute immediately after stent placement (when there has been no endothelialization over the stent) and decreases with time. The opposite is true for in-stent stenosis which takes time to occur.

The introduction of drug-eluting stents (DES) has dramatically decreased the incidence of in-stent stenosis. HOWEVER, as drug eluting stents are slower to endothelialize, the risk of in-stent thrombosis (without medication) is higher.

Dual antiplatelet therapy (usually aspirin and clopidogrel although prasugrel or ticagrelor can be substituted for clopidogrel) is of paramount importance after placement of any stent to prevent in-stent thrombosis. However, the duration of therapy is much longer for patients with drug-eluting stents. How much longer? In general, patients who get bare metal stents (not drug-eluting) need dual antiplatelet therapy for a month. Patients with drug-eluting stents are placed on dual anti-platelet therapy for one year or longer.

There is some belief supported by clinical studies that the latest generation of drug eluting stents may require a shorter duration of dual anti-platelet therapy but the current guidelines recommend at least one year.

In either case, after the clopidogrel (or prasugrel or ticagrelor) is stopped, aspirin is still continued indefinitely.