As a non-custodial parent who has joint legal custody, what can u do if your child's doctor wants him to take meds for ADHD and you disagree?

My son had some trouble in school at the beginning of the year with his behavior in class, but he has done much better recently. The school recommended that he see his doctor to be screened for ADHD. The doctor diagnosed him, and prescribed medication. His mother doesn't agree with the diagnosis but she is going along with the doctor. I completely disagree with the diagnosis, as there are no signs of ADHD. I do not want my son to take this medication. I have joint legal custody of our son, with visitation every other weekend. Is there anything I can do to stop her from giving him this medication? I believe it is going to be very harmful for him, and he does not need it.