Missed-dose advice varies a lot between medicines, so there is no single rule that fits all. For many medicines the standard guidance is to take the dose as soon as you remember, but if your next scheduled dose is near, skip the one you missed and carry on as normal. Taking two doses close together to make up for a missed one can cause side effects or overdose, so doubling up is generally discouraged.
Some medicines are far less forgiving than others. Hormonal contraceptives, certain anti-seizure medicines, blood thinners, HIV medicines and some heart and diabetes medicines have specific catch-up instructions, and missing them can carry real risks. For these, follow the dedicated missed-dose section in the patient information leaflet rather than the general rule.
The most reliable source is the leaflet inside your medicine box or a quick call to your pharmacist, who can give advice tailored to your exact drug, dose and condition. If you frequently forget doses, talk to your doctor or pharmacist about reminders, pill organisers or a simpler dosing schedule.