The most common cause of treatment failure in both human and veterinary medicine is the failure to ensure that the dose is adequate and is administered at the appropriate frequency for the appropriate duration of time. As hard as it might be sometimes, there really are important reasons why some drugs are to be given four times per day instead of three. If a horse or your individual situation does not allow for frequent administration of a drug, be sure to point that out to your veterinarian, but keep in mind you might be trading convenience for a less effective drug. The indiscriminate use of medications can do a great deal of harm, and the administration of medication to your horse should be taken very seriously. I might be a bit hard-line, but I am generally of the opinion that no medications or combinations thereof should be administered to your horse without some form of consultation with your veterinarian.
All drugs have the potential to be toxic if used in an indiscriminate manner.
There are drugs that can be toxic at the therapeutic doses if predisposing factors exist, and it is necessary to know what side-effects to look for in case there is a reaction. This includes vaccines. There are many people who are administering vaccinations to their own horses. If you are going to take on this responsibility, you also must take on the responsibility of learning about what can happen if something goes wrong. If you are doing your own vaccinating and de-worming, discuss this with your veterinarian and be prepared to recognize and deal with any complication that might occur.