Aspirin, codeine phosphate and paracetamol are such common over the counter painkillers (analgesics) that people take them almost without thinking.
But, as with all drugs, aspirin can be dangerous and can be potentially lethal if given to a child younger than 12. Children's aspirin has been directly implicated in causing Reye's Syndrome, an acute illness which results in inflammation of the brain, fever, vomiting, progressive coma and fits.
Pregnant woman and breastfeeding mothers should also avoid the drug because it crosses the placental barrier and can be excreted in breast milk.
Common side effects include epigastric (or upper abdominal) discomfort, and asthma like allergic reactions, skin and nettle rash. Perhaps the most common, though, is stomach bleeding and inflammation. Gastric haemorrhage has occurred, and special caution should be taken after an operation (particularly a tonsillectomy or circumcision) because of the increased risk of haemorrhage. It's even best to avoid aspirin for a week before surgery.