The lip twitch can be effective if you know how to use it correctly, which requires knowing the timing of the associated endorphin effect. Just as sedatives take time to work, it also takes time for endorphins to take effect when a twitch is applied, and the effect of the endorphins also wears off within a specific time frame just as sedatives do.
It takes about three to five minutes for the endorphins that are released to reach effective levels, and in the meantime, the discomfort the twitch causes may range from a minor annoyance to completely intolerable in the horse's view. If you begin whatever procedure you want performed before the endorphins kick in, the twitch will actually be adding to any discomfort, even if it does provide some distraction.
If you wait the three to five minutes to get the desired endorphin effects (pain relief and relaxation), realize that those effects will only last for about 10 to 15 minutes until the available endorphins are depleted.
Endorphin levels then drop suddenly and rapidly to below normal baseline levels. This means that even the "natural" anesthesia that is normally physiologically provided during pain responses is absent. The pain from the twitch will suddenly be multiplied, and if the procedure being performed is painful, the horse will be ill prepared to cope.
So, if you use a twitch, it is important to wait until the desired effect is observed, and don't plan to use a twitch for procedures that last more than 10 to 15 minutes.
It is never advisable to grasp an ear to twitch a horse, since not only can it lead to ear shyness, but there are multiple nerves at the bases of the ears that can be permanently damaged.