Are you sure people are taking the wrong stuff or too much of it? I have seen no data or even anecdotal evidence to support such a statement.

The horse paste itself is clearly marked by weight, which if calculated is shown to be identical to dosages for humans. It makes it somewhat foolproof.

The literature suggests that it is well tolerated in humans. This site suggests it takes 10-20x the recommended dosage before it becomes meaningfully toxic for dogs. All the literature I have seen suggests it is really difficult to overdose on Ivermectin (not that you should try).