In dealing with horse owners and their concepts of treating horses over the last 45 years, I have noticed some common tendencies that I'd like to address. Two categories of drugs commonly used on horses are antimicrobials (commonly referred to as antibiotics) and anti-inflammatories. My following comments are generalities and not intended as advice on how to treat specific cases.
The common problems I see with people using an antimicrobial drug are dosage, duration of treatment and expectations. Efficient use of antimicrobial drugs is first of all based on dosage. These drugs have a specific amount to be given based on body weight and time interval between treatments. Using less than the right amount or irregular intervals results in poor results; excessive amounts can be injurious to the horse.
Secondly, stopping the use of an antimicrobial at first sign of improvement can result in relapses and complications. Most injections must be treated for several days minimum. Some infections, such as an internal abscess or bone infection, must be treated for a month or more. Short-duration treatment can be used to prevent infection, but not to cure an established infection.
An example of this scenario would be a horse receiving antibiotics for just a couple of days with a surgical procedure to prevent infection. As for expectations, one must first be sure the problem indicates antimicrobials before just giving them.
In using anti-inflammatory drugs, the most common misconception I've seen historically is expecting results too quickly. The cornerstone drug in this category in the horse has been phenylbutazone ("bute"). I have been at ropings where someone comes to me and asks that I give their horse a shot of "bute" intravenously so it will act fast. This drug is not a pain killer like morphine. It acts by blocking the production of the products of inflammation that cause the pain. Therefore, this drug has to be in a horse's system for several hours before it starts to elicit a response. The drug will actually peak in effectiveness after a few days of treatment.
Excessive amounts of this drug can cause dangerous side effects such as gastric ulcers, so one should always use the minimal amount for the desired effect. One or two grams per day are usually well tolerated by most horses. I think these drugs are best used in the performing athlete for chronic problems such as osteoarthritis. I don't think it wise to mask pain from an acute injury and continue to use the horse.