I hear this question several times a week, so I thought I would expand on the subject! Whether your horse is on rest with hand walking, gets anxious during trailering/farrier/veterinary visits, or needs some edge taken off for riding- there are literally hundreds of options to try! Let's start with the different categories of calmers; arranged in order of efficacy.
If the horse needs a mild calming effect, I'll typically recommend a magnesium or herbal product with tryptophan, such as Quietex or Quiessence. There are lots of combinations of other ingredients including valerian root or Thiamine/Vitamin B1. An alternative is Mare's Magic- made of raspberry leaf extract. Surprisingly, I have had good results in geldings as well. Just remember (as with a lot of the below options- these are not all show safe, so be sure to check prohibited substances listed by your horse show regulatory body). If you're using a paste, closely follow the dosing instructions to allow enough time before the anticipated activity. Once a horse gets excited or anxious, it's far more difficult to get them to calm down!
At the next level, I will use Platinum Gentle for horses that are on stall rest with or without hand walking because it offers a reliable calming effect for most horses. Platinum Tranquility is the next step up for nervous horses. Even though these are costly options, preventing further injury or averting a riding accident can be a very smart investment!
A rapidly emerging market is cannabis therapy using hemp products. Horses seem to be very responsive to cannabidiol (CBD) in cases of significant anxiety. It's a safe alternative to sedatives, which we'll discuss next. One option is a hemp paste for horses by VetCS. Some hemp products contain THC in them, so make sure you get CBD products from a reliable source as USEF/FEI tests for THC.
Let's transition to prescription based products. A very reliable calming option is Zylkene. It's a milk protein product that is very effective and time to onset is as quick as 24 hours after the first dose. This can be cost-prohibitive for long term use, but provides a dependable option for very nervous horses. Prozac and other medications (Gabapentin, hydroxyzine) can offer calming effects although that's not their primary mechanism of action. Be wary of intravenous forms of magnesium being given to show horses to calm them, this route is not without serious risks!
The last group of medications are reserved for highly anxious horses, because they are truly tranquilizers. Acepromazine is the classic go-to for injectable sedation and can be given IV, IM, or by mouth. This requires 30-40 minute delivery prior to activity, much like an oral paste. The effects are dose-dependent, and some ataxia (unsteadiness) will ensue at the higher doses. Another short-term injectable medication is SediVet (Romifidine hydrochloride) but it's extremely difficult to get a hold of right now. It produces a more reliable sedation with less ataxia than Ace. There are several long-term tranquilizers that can be used in horses, but they are not without significant risk. Long term sedatives can cause significant neurologic behaviors that cannot be reversed and you guessed it: the horse has to work through severe side effects for 1-2 weeks! This group of long term sedatives are used rarely; however, it is hard to tell if a horse has been given one of these medications, which makes a drug screen an important testing modality if you're looking to buy a horse.
You can see why I thought this was an important topic - there are so many options! As always, we are happy to answer any questions you may have on safe methods to calm your horse(s). Happy trails~