Dogs who are reactive display aggressive symptoms for 1 of 2 reasons:  fear or frustration.  Sometimes, it's a combination of both.  Usually, the issue is primarily on-leash, not off-leash.  Why?  Because the dog's fight or flight response is inhibited by being on-leash.  They cannot go after what they're excited to see, nor can they run away from what they're afraid of.

Terminology review:

• Trigger:  the person, place, or thing your dog reacts negatively to.
• Threshold:  the space between the dog and the trigger required for the dog to remain in control of themselves.  For sounds, the threshold is not so much space as volume.  We always want to work on the dog's triggers under threshold, below the point at which the dog is upset.
• Trigger stacking:  allowing the dog to experience distress from multiple triggers in a short period of time.

When we meet to address your reactive dog's behavior issues, my solution will be customized to you and your dog, based on my observations in the first session.  However, these are my go-to behaviors to train for reactive dogs:

• Emergency U-Turn.  In order to be successful, dogs must remain under threshold.  The number 1 tool in your toolbox for your reactive dog is space.  However, the dogs will often lunge and pull toward the trigger and not want to get away initially.  The emergency u-turn teaches the dog it is highly valuable to voluntarily walk away with you.  I use the cue "Let's go."
• Mat + relaxation protocol.  I will teach all reactive dogs to go to a mat, lie down, and stay until released.  Then begin Dr. Karen Overall's Mat Relaxation Protocol.  Once you're up to level 6 or so, we're ready to take it outside and begin using the mat as a safe zone for dealing with triggers.  By providing structure, you give the dog a way to tolerate their triggers in their space, and also give you polite cues of permission for closeness.  If the dog breaks the mat stay, you know the trigger is too close.  There is no need for aggression.
• Leave It.  Especially for dogs who are frustrated, this behavior is essential.  By being able to keep themselves from going for something they want, the dogs learn how to cope with that sort of stress in a positive way.  Start with something only kind of valuable, and work up to something that really gets your dog going.  Add duration (how long they have to look at you and leave the temptation alone before being rewarded).  I use Leave It & Get It in tandem with toys, or Leave It and reward with something else with food.  That way, the dogs can experience the relief of getting what they want, or understand it's just as valuable to never get the initial temptation.
• Watch Me.  This is absolutely essential.  Reactive dogs are often hyper-vigilant, constantly scanning the environment for triggers.  This does no one any good, and increases the stress level in the dog.  By teaching the dog to default to focusing on the owner, we give them an alternate behavior to fill the void scanning leaves behind.  We also teach the dog good walking manners by building duration with Watch Me and adding motion.  A dog pulling on the leash and forging ahead cannot comfortably look at their owner on a walk.

If your dog is frustrated, please consider investing in a flirt pole, or at least playing tug frequently with your dog.  This is the best way I have found for most dogs to vent their frustration.  If your dog tends to redirect onto the leash by biting it when triggered, consider using a braided fleece tug leash.  Muzzling the dog and preventing their biting does nothing to vent the built up frustration.  I only recommend muzzling if the dog is a threat to themselves, their owner, or other animals.

I like to give dogs a detox period prior to starting work.  For at least 1 week, avoid all scenarios that may trigger your dog.  One major incident can release the stress enzyme, cortisol, into your dog's system that will stay in their bloodstream for up to 7 days.  So if your dog got really upset on Thursday and our session is on Friday, we'll be attempting to combat that day's stresses and Thursday's as well.

Please avoid trigger stacking as much as possible.  Never try to tackle a dog's fear of the vacuum, frustration around other dogs, and wariness of strange people in hats all in the same session.  Focus on one trigger per session.

This is a touchy subject, and for good reason.  There is a lot of stigma around behavioral medication for people and dogs.  Medication is seen as a band-aid and overused.  As someone who suffers from PTSD, I had a period of about a year where I personally took prescription medication (Ativan as needed for anxiety, and Lexapro for depression) after a car accident.  I've also had a dog who warranted being on medication for fear and aggression.  It took me 3 years to come around to the idea, and those 3 years were VERY difficult, even with me as a professional trainer.  I finally sought the opinion of a board-certified veterinary behaviorist, who immediately said my dog should be on medication.  I finally acquiesced, and that medication gave us 2 more years together, and they were the best years.

How do they work?  Prozac (or the generic Fluoxetine) is an anti-depressant in people, and can also help dogs suffering with fear and anxiety.  The aggressive symptoms they display become less intense or go away entirely.  The disadvantage is this medication has a loading period of a few weeks to get into the dog's system, and playing with the dose is necessary to find the magic.  Dogs on the right dose do not act like zombies or seem in any way "drugged up."  They are simply benefitting from the medication and doing better.  Prozac requires a weaning off period to avoid withdrawal.  Trazadone helps with fear and anxiety as well, and has a mild calming effect at the right dose.  The advantage is it is fast acting and there is no withdrawal to consider.  The disadvantage is the calming effect and that it is situational, you must keep giving the dog the medication for the effects to be seen, it does not build up in their system like Prozac does.

I'm never trying to "load your dog up with drugs."  If a dog warrants being on such medication as Prozac or Trazadone, the goal is ALWAYS to wean them off after the medication has had time to provide its service.  That period can be between 6 months and a year or so.  Sometimes, it's purely a chemical imbalance causing the behavioral symptoms, and during the weaning off process, it is determined that the dog should remain on the medication indefinitely.  However, that is not the goal, my intention, or what usually happens.  The medication is a tool, just like a training harness.  The goal is always to wean off tools as the dog's training progresses and improves.

Generally speaking, these dogs don't have a particular traumatic event causing their fear and aggression.  I usually try to tackle just that trauma without medication.  These dogs were bred by irresponsible breeders who didn't consider temperament enough before having a litter of adorable puppies and selling them to unsuspecting owners or surrendering them for adoption.  However, even responsible breeders who do everything right can have an off puppy in a litter of otherwise happy-go-lucky, normal puppies.  When I take a history and determine the owner has owned the dog their entire life, from puppyhood, and done everything "right" but the dog is still extremely upset, no amount of training the world can fix genetics and brain chemistry.  Besides genetics, prolonged stress can also alter brain chemistry, creating neural pathways that prohibit stress management and increase anxiety and/or aggression.  If your dog is 2 years old or older, has always "been like this," and is constantly in distress, that's a huge red flag for poor brain chemistry.  It takes a very severe case for me to recommend medication for dogs under 2 years old.  Puppies are developing physically and emotionally, going through various phases -- so I prefer to wait.  

Medication as part of a Behavior Modification Plan will help the adjust chemicals in the dog's brain that control stress and make everything easier.  The dog's quality of life improves because they're not so upset by their triggers, and training will progress exponentially faster because the dog has a better frame of mind from which to learn.  I will be an advocate for the dog and recommend medication if I deem it advantageous to our plan.  

Having just been informed someone thought I was "a piece of sh*t trainer" for recommending their dog try behavioral medication (Fluoxetine, the generic for Prozac), I wanted to outline my personal criteria for recommending a prescription for medication from your vet in case it can help someone else in future:

1. The dog displays multiple triggers that create an intense reaction.  If your dog trembles in fear and/or growls and barks with aggression to multiple triggers such as everyday sights (bikes, dogs, people, etc.) or sounds (vacuum, sirens, wind through leaves, etc.), the quality of life for your dog must be very low.  If they are easily triggered by multiple everyday things, they are in a constant state of stress.  That isn't fair to the dog, but we can't remove these items from their lives realistically; especially not long enough for counter-conditioning protocols to begin to take effect.  Behavior modification, done correctly, takes a long time and lots of patience for all involved.  Rushing emotional healing never works and usually backfires.
2. The dog displays severe aggression (especially human aggression) and may have a bite history, especially if the bite was a level 3 or higher (drew blood).  If the only reason your dog doesn't have a bite history is because it's in a muzzle or on a leash, this is a very similar issue of quality of life as with the dog in #1.  I always try a session or 2 to tackle these cases with counter-conditioning protocols to see how much progress can be made without medication. (I am just finishing up a case where a dog was slated for behavioral euthanasia at a shelter for being "un-rehabilitatable." He was saved by a rescue despite his bite history, and adopted by a wonderful family committed to helping him overcome his fears.  His food drive is so intense, we were able to make him safe and manageable without medication in 5 sessions.)  If after a couple sessions I'm not seeing the minimum level of progress I would expect, given my experience with hundreds of other cases, I will recommend medication.
3. There are children involved, or there is a quick timeline requirement.  If the dog is living with children and their safety is at risk, I will be more likely to recommend medication.  Sometimes, people need to travel and need the dog better enough by a particular date, or there is a dog in a rescue or shelter with extreme behavioral issues not getting adopted and their time is running out.  I will be more liberal in recommending medication in these circumstances.  Shelters often put aggressive or fearful dogs on Trazadone to make them more adoptable since it's such a high-stress period of life even for a behaviorally healthy dog.  That includes dogs in foster homes, because they've been uprooted from whatever their normal life was, and put into a completely different one.  That's extremely unsettling, even for people who can be told why it's happening.
4. The dog suffers from separation anxiety and is mutilating themselves.  Some dogs who suffer from severe forms of separation anxiety will turn their stress on themselves and chew wounds into their limbs.  I knew a client's dog who chewed off their own toe and ate it.  Another dog chewed off the tip of their tail.  Or they will try to break out of whatever containment they are in (a crate, for example) and end up injuring themselves in the process.  I will immediately recommend medication in these cases, for the dog's well-being and safety.
5. We have personally tried, or you have previously tried with another trainer, to solve things with standard behavior modification protocols, and perhaps calming supplements like lavender spray, L-carnitine or CBD, but it's just not working.  If we've done about half a dozen sessions together (or you've had them with another trainer using similar methods) and seen no significant improvement despite doing all the required homework, it would likely be irresponsible to continue without suggesting medication.  Sometimes, a dog is so shut down, we can't tell their level of stress purely by observation.  Dogs are designed to hide weakness, and it may take a few sessions to discover just how much a dog is unstable mentally.
6. The owners' level of involvement.  If the owners are very busy and don't have a lot of time to dedicate to working with the dog everyday, but are committed to keeping the dog, I may recommend medication to speed things along.

I will ALWAYS respect the wishes of the owner.  There is no judgment either way the owner decides.  If you decline medication, that is your right.  If I believe it possible to do safely, we may continue on with training as best we can.  If I feel very strongly the dog should be on medication for safety reasons, I will terminate the training relationship.  I don't report owners as neglectful to animal control for declining behavioral medications.  This is a very personal decision, but my priority is 100% the quality of life for the dog.

When I recommend medication, you can get that medication from your veterinarian or a veterinary behaviorist.  I am not a doctor, so I cannot prescribe medication.  It is then the responsibility of the veterinarian to also determine the dog warrants medication.  I have yet to have a veterinarian turn down a client after I make the recommendation.  Note:  this means that 100% of clients who followed my recommendation to seek medication for their dog have been approved by their veterinarian.  However, there are veterinarians who don't like to prescribe these medications and refuse to even entertain the idea -- you can go to another vet and ask for a second opinion.