While it's not likely that you will cause problems by over-dosing ivermectin, pyrantel or fenbendazole a little it can happen. Especially if there is some pre-existing damage to the liver, central nervous system or kidneys or if your horse is eating certain plants in the pasture in the case of ivermectin. Moxidectin requires more careful dosing than the other dewormers, but it still has a fair safety margin.
Neurological deficits including loss of balance, blindness and depression are symptoms of overdose of ivermectin or moxidectin.
You are always better off calculating your horse's weight or dosing a little higher than you "think" your horse's weight is because under-dosing increases the possibility of the parasites becoming resistant to the drugs we have to use.
Licensed Veterinary Technician