Prescription medicines are the real problem. Anyone in the household who depends on a daily medicine should have more of it than the days they can expect to be without a pharmacy. Ask a doctor or pharmacist how to build a small margin lawfully where you live; the answer varies by country and there is usually one.
Write down, on the paper with the contacts: name of the medicine, the dose, and what it is for. Not the brand of the box you happen to have. In an unfamiliar pharmacy or another country, the generic name is what gets it dispensed.
The box itself, for what a household actually meets. Plasters and sterile dressings in more than one size, a bandage that can apply pressure, adhesive tape, scissors, tweezers, disposable gloves, antiseptic, painkillers, rehydration salts, any antihistamine somebody needs, a thermometer.
Check it once a year on the same day you talk through the plan. Things expire and things get borrowed.
A first-aid box is what a household keeps. It is not a substitute for calling for help, and nothing in this course is medical training.