Here in the US heartworm preventative medications are sold as prescription drugs. Depending on where you get it and the size of your pet, a six-month supply normally costs from around $30-60. Honestly, this is a good price considering that treating heartworm disease can easily run $800-1000. Suddenly $5-10 per month doesn't sound so bad. But even with that minimal cost many people try to find ways to do it even cheaper and avoid having to do the annual heartworm testing required by most veterinarians.
Most horse antiparasitic medication is easily available over-the-counter at farm supply centers, and people will try to use this instead. On first glance, it seems like a good idea. The main ingredient in the most common heartworm preventions is ivermectin. This is also a common ingredient in many horse dewormers, and these equine medications are usually much cheaper. So some pet owners will purchase horse medications and give it to their dogs as heartworm prevention. Theoretically there isn't anything inherently wrong with this method. However, there is a significant danger that most people aren't aware of. In high enough doses, ivermectin can be toxic.
A single tablet of ivermectin-based heartworm prevention for the largest category (51-100 lb dog) contains around 0.27 mg of ivermectin. Horse medication is much more concentrated since it is designed to treat a 1200 pound animal at a dose of around 0.09 mg per pound. Dosing based strictly on weight, a 100 pound dog would then receive 9mg of ivermectin rather than 0.27mg! This 33 times the recommended dosage!!!! If someone is not VERY, VERY careful with the dosage, it's easy to overdose a dog using horse medicine. And this happened in my clinic yesterday.
A small dog came in very lethargic after having been given horse dewormer as heartworm prevention. The dog was sedated and showing slight neurological signs, classic symptoms of ivermectin toxicity. The owner's neighbor used horse dewormer as heartworm prevention, so she decided to do it herself. Unfortunately she gave far more than was needed and the dog suffered because of it. Thankfully, the toxicity was mild enough that the dog will recover. In many cases of ivermectin overdose if you stop giving the medication immediately the symptoms will resolve. But in severe cases you can see severe neurological disorders that can be very harmful.
There is also a problem in certain breeds of dogs, mainly herding breeds (collies, border collies, and related breeds). These dogs have a genetic quirk where they lack a gene to produce an enzyme that will break down and eliminate ivermectin from the body. This abnormality makes these breeds especially sensitive to ivermectin. The doses found in heartworm preventions are low enough that there isn't a concern, but when using doses higher than this you can run into toxicity very quickly.
The bottom line is that you should only use medicines that your veterinarian specifically recommends. Doing otherwise could lead to harm for your pet and potentially expensive veterinary visits to treat unwanted consequences.