When used as directed, BC powders are not bad for you. However, like any medication, misuse can have harmful side effects.
BC powders are made up of aspirin and caffeine, neither of which is bad for you in small quantities. BC powders are often used for temporary relief of pain such as body aches and headaches.
WebMD points out that prolonged use of aspirin or aspirin-containing products may lead to stomach problems such as gastritis or an irritation of the stomach lining. Nausea and vomiting are also possible if more than the recommended dose has been consumed.
Caffeine, the second component in BC powders, may cause insomnia, irritability, nervousness, and possibly nausea. These symptoms will dissipate over time and they are less severe when dosage suggestions are followed.