Is the dose too high? Significantly higher than you agreed to? Significantly higher than the original doc prescribed? What are the side-effects of Tenex?
Who cares if she tells the school the sky is green.
What is the dosage?
Is it significantly higher than what you were in agreement with?
What are the side effects of Tenex?
In other words, to an unbiased third party, how is the child being harmed by her doing this?
That is the argument you need to build. Again, who cares what her mouth says. This isn't about anything other than your showing she's putting the child at risk.
You have a case where you can argue she's ignoring joint consent, for which you'll get clarifying orders, her slapped, and potentially a contempt finding.
If you want real change, again, how is she putting the child at risk?