Side effects can occur with every drug prescribed off label. That's why doctors want to be the ones to prescribe them and monitor instead of going back to the days before accepted trans care where trans people, including minors, self medicated or killed themselves. Puberty blockers like Lupron in particular, which is often the extent minors are allowed to even go to until the age of medical privacy and consent, have been studied and widely used for decades with even cisgender kids and found to be overwhelmingly safe.

Again, to erect more barriers to stop fringe cases ultimately limits care for exponentially more people who did need it.