The introductory paragraph is the part of the essay in which you state what you will focus on and why. What is mostly suggested is that you start your essay with one catchy sentence that will attract the reader's attention, and to follow such sentence with the description of what your essay will be about.
Writers cannot assume that their readers are in any way familiar with the main topic, so you should explain in very simple terminology what is aspirin, what is it used for, and who uses it (preferably with actual, researched demographics that give validity to your statement).
A possible example of an opening statement, followed by the main idea could be as follows:
Imagine a world where headaches and toothaches were incurable; could we really live without aspirin? Scientifically named acetylsalicylic acid, the commonly-known drug aspirin is a naturally-occurring chemical substance that can be found in plants such as the roots of meadowsweet. It was officially discovered by German scientist Arthur Eichengrün, and its analgesic usage as a pharmaceutical has successfully relieved millions of people from pains, headaches, and even thrombosis, since its first production as a drug in the 19th century.
Notice how the opening statement is connected to the main idea, and you can probably take away additional or unnecessary details from the example provided, depending on who is your audience, and on what type of essay you are planning to write.
An eNotes link on "aspirin" is included below so that you can get more information as well. Happy Writing!