(Dunno how they are sold in your country) a bottle of nitroglycerine or similar drugs, the instructions to which you know by heart, similar to Harry's preparations? Considering that the probability of you encountering a stranger who has an emergency should be higher than the p of only you having it, unless there is a common cause. In case there is a common cause, well. Bring a gun?.. (At least it is small.) A notebook seems also a useful thing to havehave, with a pencil attached.
Bring a gun?
This needs to be qualified by lots of clauses like local law, necessary practice... But maybe a smaller alternative: pepper spray?
This thread is for asking any questions that might seem obvious, tangential, silly or what-have-you. Don't be shy, everyone has holes in their knowledge, though the fewer and the smaller we can make them, the better.
Please be respectful of other people's admitting ignorance and don't mock them for it, as they're doing a noble thing.
To any future monthly posters of SQ threads, please remember to add the "stupid_questions" tag.