system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do pills such as Paracetamol come in a nice number like 500mg, But others such as Aspirin come in numbers like 81mg? why not 80?
gpt
For some medications it's because they come in both micrograms and milligrams so they make one of them slightly off so people don't accidentally take 1000x the usual dose.  Baby Aspirin comes in 81mg because it's 1/4 as much as normal Aspirin which is "5 grain" or 325mg.