PSA: REVIVES SHOULD NOT BE EATEN INCLUDING BY POKEMON
Revives are what are called sublingual medication. They are held in your mouth under the tongue or in the cheek and dissolve over time. Once a pokemon recovers, the revive should be spat out and discarded.
Colloquially, people like to say they've 'fed' their pokemon a revive, but in professional practice, the terminology of 'take' or 'administer' should be used to avoid confusion.
If a revive is swallowed whole like a pill, only a small portion of the medication will be dissolved in the mouth and make it to the bloodstream, so it will be ineffective. If a pokemon (or person) is unconscious, there is a high risk that they will also choke on the revive.
If a revive is crushed or chewed then swallowed, there is a risk that a much higher dose than needed will be released quickly. This could lead to toxicity and severe side effects.
And, I can't believe I have to say this, but revives are not designed for use by humans. The pathway the medication takes in pokemon to produce the rejuvinating effect requires several enzymes humans do not have, and those enzymes help metabolise the active ingredient into a harmless byproduct that can be easily excreted. At high concentrations, the active ingredient can be very toxic. As humans cannot break down the active ingredient, the concentration can quickly build up to harmful levels.
The active ingredient in revives can cross the blood-brain barrier, leading to side effects of feeling lightheaded, dizzy, confused, anxious, irritable, and so on. This is why it's not recommended to send your pokemon back into battle if they show any of these symptoms - their cognitive functions may be impaired, so they run the risk of getting heavily injured in battle, even if that is not the opponent's intention. If you're a human, swallowing a revive whole is a choking hazard, but you'll probably be fine. However, sucking on them or crushing them carries the risk of permanent brain injury.