Bisphosphonates is a class of drug which prevents the loss of bone density in the racehorse.
These are substances are prohibited substances within the Rules of the NHA:
73.4.1 Substances capable at any time of causing an action or effect, or both an action and effect:
… 73.4.1.6 the musculoskeletal system;

There is now an allowance by which older racehorse are allowed to be treated for osteoporosis (which results in low bone density bones which are brittle and could break) which do occur with older horses.

The formal International Federation of Horseracing Authorities (IFHA) statement within Article 6 of the International Agreement on Breeding, Racing and Wagering is:

Any bisphosphonate is not to be administered to a racehorse:

• under the age of three years and six months as determined by its recorded date of birth; and
• on the day of the race or on any of the 30 days before the day of the race in which the horse is declared to run.

The bisphosphonate product administered must be licensed for use in horses in the country in which it is being used, and be administered in accordance with the label instructions.

There must be a diagnosis determined by a veterinary surgeon that supports the use of a bisphosphonate as an appropriate treatment, and such treatment must be administered by a veterinary surgeon.

There is the requirement that 

• Only local registered horse-approved drug substances are used.
  There is only one such licensed for use in horses in South Africa, called Tildren which contains Tiludronic acid (also known as Tiludronate).
   Other, locally available human preparations and non-licensed and non-registered products and other bisphosphonates may not be used.
• That the horse must be an older horse (not still a growing horse).
  the racehorses must be 3.5 years old at least (the age when osteoporosis may become problematic).
  There must be control by means of a veterinarian and a formal treatment regimen from a veterinarian (according to the recommendations for the     specific product).
• That younger racehorses (below the age of 3.5 years) may not receive this treatment (as osteoporosis treatment should not be required).
• It is specified that the use of such medication must be prescribed and controlled by a veterinarian and that such treatment must be well-detailed by   the veterinarian in the VTR of the corresponding horse and trainer.

It must be noted that

• a positive finding will result when it is detected in a racehorse younger than 3.5 years.
• that a positive finding will result if it is detected in a racehorse older than 3.5 years - if an official, corresponding diagnosis and prescription is not in     place.
• that a positive finding when result if it is detected on a race day in a racehorse older than 3.5 years - if a 30 day withdrawal period was not complied   with.
• There must be an official diagnosis determined by a veterinary surgeon that supports the use of a bisphosphonate (Tildren) as an appropriate   treatment.
• Tildren must administered in accordance with the product label instructions.
• Tildren treatments must be administered by a veterinary surgeon.