While your primary application will land you general consideration across schools and get your foot in the door, the secondary application is what will set you apart at a school and make you eligible for receiving an interview.
Secondaries, unsurprisingly, take a lot of time to write. You often have to answer as many as 6 or 7 essay questions, or for some, describe your life in one long essay. If you are in school, avoid taking difficult summer classes while working on secondary appplications.
Everyone is going to tell you this over and over again, and we really recommend listening to this piece of advice. This is because it can be seriously overwhelming to receive many secondaries all at once in your email inbox (although the first few are quite exciting).
Here are secondary application essay prompts from medical schools in previous years:
https://www.prospectivedoctor.com/medical-school-secondary-essay-prompts-database/
Many schools do not change their prompts, while some do. You may realize there is a pattern – a lot of schools ask similar questions. Often times, if you write the backbone of a common prompt, you can fill in the blanks with school-specific information. This helps make pre-writing a lot easier and you won’t have to gruel over writing hundreds of individual essays!
The general rule we've heard is to try to keep the time between receiving the secondary application and submitting it to under two weeks if you can. If it has been more than two weeks, consider whether or not you want to submit that secondary school. Maybe you have been sitting on it because you have doubts about attending the school.
As a pre-medical student, you have probably applied to a number of clubs and scholarships in the past for which you had to write essays. We recommend keeping some of those handy so you can refer to them when writing your secondaries. It also helps you track how you have changed over time - you might begin to realize how much you have grown since your very first applications in your freshman year of college!