Aspirin crystals, polarized light micrograph. Aspirin (acetylsalicylic acid) is an analgesic (pain-relieving) drug. It is a non-prescription drug and is used to relieve general pain such as headaches. Aspirin is also used to thin the blood, which reduces the risk of blood clots that can cause heart attacks and strokes. Magnification: x25 when printed at 10 centimetres across.