An immediate-release drug is a formulation that releases all of its active ingredient soon after it is taken, so it works quickly and wears off within a few hours.
Short-acting methylphenidate and dexamfetamine are examples used for ADHD. A dose lasts a few hours, so it may be taken more than once a day.
Immediate-release medicine is useful when flexibility matters: a top-up dose in the late afternoon, a trial of how a medicine feels or a day when the medicine is needed for only a few hours. The trade-off is remembering several doses, which ADHD itself makes harder.
Some adults take a long-acting medicine in the morning and an immediate-release dose later in the day. Your psychiatrist works out the combination with you during titration, the period of adjusting the dose.