The treatment of PPID itself is relatively straightforward. You administer a daily tablet containing pergolide, which mimics the action of dopamine, a hormone that is insufficiently produced in horses with PPID. This helps regulate the dysregulated pituitary gland, reducing the clinical manifestations of the disease your horse experiences. Despite the simplicity of improving the health and quality of life for most horses with PPID, medication adherence appears to be low.
Medication adherence: the extent to which the horse owner or other in charge administers the right medication, in the right dose, at the right time and in the right way.
Scientific research
A large study from 2021 examined the clinical records and blood test results of 110 horses, along with owners’ explanations. It found that more than half of the owners (52%) did not adhere to the vet’s recommended dosage or the prescribed frequency of administering the drug [1].
The older the horse, the less likely the owner’s adherence to the vet’s advice. This was particularly true for horses aged 26 years and older, where medication adherence was only 17%. This attitude mirrors the lower willingness of owners to consistently vaccinate and deworm older horses [2], but this aside.

But that doesn’t apply to you and your horse, does it?
Pergolide as a victim of its own success
Once we start treatment with pergolide, the amount of ACTH (the pituitary hormone of primary interest) decreases immediately and significantly [3]. There is a risk that this may compensate for or perhaps even worsen low medication adherence. When the effect of medication is so strikingly positive, it is tempting for some people to reduce the dosage. Only, the drop in ACTH levels gradually diminishes over time. This makes the drug a victim of its own success, as owners will be less motivated to revert to the original dosage.
Without retesting, the status of the blood values remains unknown. The study’s findings also indicate that owners who are less compliant with the medication are less likely to have blood tests conducted again.
Shetland ponies are a story in their own right
The study also found that medication adherence and the willingness to check treatment effectiveness with a follow-up blood test were significantly lower among owners of Shetland ponies compared to those of other breeds. The researchers suggest that this could be because shetlanders can sometimes be a tad stubborn and therefore more difficult to ‘convince’ of the importance of a daily dose of pergolide. Do we hear some shetland owners chuckling in agreement there?

(Photo: Rhys Fradley)
The modest size and lower body weight of Shetland ponies might also contribute to the issue. For example, a 150-kg pony requires only 1/3 of a tablet. Accurately dosing such a small amount can be challenging with tablets that can only be broken into halves or quarters. Struggling to administer fragmented tablets to a resistant pony every day is not an ideal recipe for consistent medication adherence either.

Blinded by blood values
Despite the fact that half of the horses received less than the recommended dose of pergolide, the researchers did not see this reflected one-to-one in the laboratory results. Simply put: in horses given less than what was prescribed, ACTH-levels still went down. Also, the clinical picture of some horses improved even when given pergolide at a dose that is scientifically not optimal [4].
Break line
This can be explained in several ways. First, a horse may require less medication than the prescribed amount [5]. Until quite recently, only one brand was available (Prascend™). Because these tablets have a single break line, it is easier to think in terms of whole and half pills. However, a whole pill may be too much for horses that need three-quarters of a tablet. Giving only half a pill could lead to under-treatment and contribute to the issue of non-adherence, though it might still improve blood counts. Fortunately, there is now another brand (Pergoquin™) that offers tablets that can be divided into quarters. If necessary, a pill cutter can also help achieve the correct dosage.

A second explanation proposed by the researchers is the sharp drop in ACTH levels that occurs immediately after starting treatment. This initial drop may make occasional missed doses less noticeable in blood tests. If medication is resumed promptly after an owner realises, from a reminder in their diary, that the vet will be visiting for a follow-up blood test, blood levels can improve quickly, masking the effects of previous non-adherence.
False positive results
Finally, there may be instances of misdiagnosis or false-positive blood test results. This is especially likely if the clinical signs are inconclusive and the ACTH level is only slightly above the critical threshold, or if the blood test was conducted at the beginning or end of the seasonal rise. In such cases, the observed improvement might not be due to the medication but rather to natural seasonal changes or other factors that can elevate ACTH levels, such as pain, stress, or exertion.
Clinical picture
Also, remember that the clinical picture is at least as important as the blood count. Relying solely on ACTH levels for diagnosis and treatment is not advisable, especially since ACTH testing only correctly identifies a horse as PPID-positive in seven out of ten cases [6].

Up that pergolide dose?
But when we look at horses whose values do not improve or even increase, another issue arises. If the clinical picture does not improve, most veterinarians will opt to increase the dose. This is partly because vets often significantly overestimate their clients’ medication adherence [7]. Instead of assuming that the pergolide does not work sufficiently, it would be wise to check first on the owner’s dedication and persistence.
If these leave room for improvement, it is important to identify the underlying reasons. Is there a general fear of drugs and possible side effects, or concerns specific to pergolide? Is the cost too high? Does the horse refuse to take its tablet, or is it difficult to administer the correct dose daily for practical reasons? Offering advice on how to overcome these obstacles is significantly more effective than simply increasing the dose.

All on the same page
Besides dedication and persistence in administering medicine, how much you and the vet agree on the chosen treatment also affects your adherence to giving your horse its medication. The treatment goal may not yet be entirely clear to you. Additionally, treatment adherence increases if you feel involved in the vet’s decision to prescribe medication. It is also possible that you find it so difficult to accept that your sweet, aging horse has a serious illness that you remain in denial for a while and therefore do not want to give it any medication.
For all these scenarios, the same advice applies: seek guidance, ask questions, and express your concerns. The more you know and the better you communicate, the better it will be for your horse.
So, now you want to know everything about PPID?
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