The use of aspirin in the EC combination was investigated due to the fact that aspirin blocks prostaglandin production, and prostaglandin E inhibits norepinephrine release from neurons.
The study looking into this combination unfortunately showed that aspirin did not increase the effectiveness of EC, except in obese women.
The use of aspirin is probably more harmful than beneficial, since the chronic use of aspirin can be damaging to the intestinal lining.
The addition of yohimbine is a much more effective combination.