Children have not been included in controlled studies of clot-busting drugs, probably owing to the low incidence of heart attacks or strokes in this population. In general, children are more sensitive to the effects of medications. Therefore, the risk of bleeding of other side effects may be higher for children.

Also, clot-busting drugs must be administered within hours of the onset of symptoms. Childhood strokes are not typically diagnosed quickly, often resulting in major, long-term disability. One study found that the average time between the onset of stroke in children and the diagnosis was almost 36 hours. To be most effective, clot busters should be administered within three hours of the beginning of symptoms.

Despite the lack of comprehensive studies of clot-busting drugs in children, low doses of t-PA have been used successfully in children with acute thrombosis (blood clots).

Elderly patients (75 years of age and older) often have an increased need for clot buster treatment because they are more likely to have a heart attack or stroke. Some studies recommend, however, that thrombolytics (following a heart attack, for example) are only used for elderly patients only if the event occurred within four to six hours. Beyond that time frame, beta blockers, aspirin or ACE inhibitors tend to be given. Other studies, moreover, show a preference for angioplasty over clot busters, provided that cardiac catheterization can be done immediately.