@storyist Not quite! These days there's fast-acting anxiety meds, 'fast acting' meds that take a little longer to kick in (though they usually last longer as a trade-off), and daily meds that need to build up in the dog's system for a couple weeks to be fully effective. How well and quickly they work are going to depend a little on the dog and situation, and generally the 'fast-acting' medication that needs a longer time to kick in is only prescribed to dogs with anxiety issues in predictable/controllable situations, like travel or separation anxiety.
Acepromozine used to be very popular for thunder phobia and the like, but is now almost never used for anxiety issues - and should absolutely never be used alone for that purpose. We now know that while it makes the animal outwardly calm, it does nothing for the underlying anxiety, so they're now experiencing the scary thing while unable to move properly. Understandably, ace often resulted in worsening anxiety for this reason. There's more options today, and they're more appropriate for actually handling fear.
(Obligatory not a vet, just saw a talk on psychiatric medication in veterinary medicine recently and found it very cool).