Learn Spanish (Spain): Deberías tomar el antibiótico cada ocho horas. - You should take the antibiotic every eight hours.

Deberías is the conditional tense of deber. While debes is a strong obligation meaning "you must", using the conditional deberías softens it to mean "you should" or "you ought to". This is the standard and most polite way to give advice or a medical recommendation in Spanish.

Why do we use tomar for medicine instead of other Spanish verbs that mean "to take"?

In Spanish, tomar is used for consuming food, drinks, and medicine. While verbs like coger or llevar also translate to "to take" in the sense of grabbing, catching, or transporting an object, tomar is the only correct verb for ingesting a medication like an antibiotic.

Does the word cada change to match plural or feminine nouns, like cadas ocho horas?

No, cada (every or each) is an invariable word. It never changes its ending, regardless of whether the noun that follows is masculine, feminine, singular, or plural. You will always just say cada ocho horas.