The disadvantages of horse wormers are:

  • Cost. Although not a major element in the cost of a horse, wormers need to be given a number of times each year, so the expense does add up. It is often cheaper to buy from the internet rather than your local veterinarian. Alternatively, if your horse is boarded at a professional stable, having the owner do a bulk purchase for all the horses can result in a better price.
  • Difficulty. Many horses object to the smell and taste of worming medication. Fortunately (see How to Worm), there are a number of techniques to make this easier.
  • Damage to Health. If a wormer is given in excessive amounts (e.g. double or more the recommended dosage), like any overdose, it can have negative impacts on the horse's health. Likewise, if it is given too frequently (once every two months is normally the maximum, although in cases of horses very badly infected by worms veterinarians may recommend more frequently to begin with). Furthermore, many people have concerns about the potential long term affects of these chemicals on internal organs.
  • Foals, Lactating Mares, Ill Horses. One needs to be careful about worming very young horses, very old horses or ill horses. Such horses have reduced resistance to the chemicals in wormers and consequently one should seek medical advice for such horses as to the most appropriate worming options. One also needs to be wary of worming lactating mares (mother horses which are giving milk to foals) as a portion of the chemicals can be passed onto the foals in the milk.
  • Resistance. Like are medicated parasites, worms are developing resistance to the drugs used against them, reducing the effectiveness of such drugs. For this reason, any worming programme should alternate medication types, to counteract drug specific resistance.

Due to these factors, many people choose to worm their horses less often, often using Alternatives to Wormers in order to reduce the frequency with which worming medication is required.

Advantages of Wormers

Wormers are a quick, easy and effective way of keeping the number of worm parasites to an acceptable level. Without the regular use of such medication, most domesticated horses will have excessive levels of worms. The disadvantages of this include:

  • Expensive to Feed. High levels of worms result in much of the food being eaten by the horse being consumed by the worm parasites, rather than the horse. As a result, the horse needs to be fed substantially more, with a corresponding increase in feed costs.
  • Starvation. In extreme cases, the worms consume so much food that the horse becomes malnurished, loses weight and can even starve to death.
  • Cross Infection. The more worms a horse has, the more eggs and larvae the worms are producing, which are deposited on the field with the horse's manure. This results in higher infection rates and consequently higher worm levels in other horses sharing the same pasture. For this reason, it is advised that all horses sharing a pasture be wormer at the same time and on a regular basis.
  • Damage to Organs. Worms live not only in the intestines, but also migrate through the lungs and various internal organs (which organs are affected depends on the worm type). There movements can damage these organs, as well as the intestinal walls. If the levels of worms are high, this damage can be correspondingly high, leading to excessive bleeding or various illnesses (such as colic), either of which can be fatal. 


The disadvantages of wormers has resulted in many people choosing to minimize their use. However, the disadvantages are clearly outweighed by the overall benefits to the horse's health of their regular use. In fact, the widespread use of wormers is a major factor in the modern increase in the average life expectancy of domesticated horses.