Why does adhd medication cause tics in children?

It depends.: It depends on the child, and on the medication. Some adhd medications are stimulants, and would tend to make tics worse in anyone. Other adhd medications should not worsen tics. It's best to discuss this with your doctor.

Pharmacology: Tics are repetitive, involuntary, stylized, motor movements involving a single or complex group of muscles. Chemicals that are stimulatory in nature, such as caffeine or methylphenidate, may excite muscle fibers to contract more easily. Brain control of muscle firing may also be increased with these substances. Tics are worsened by stimulants, not caused by them. See my health guides on panda's.

Increases dopamine: One cause of tics is related to an excess of brain dopamine, an essential brain transmitter. Stimulant medications increase brain dopamine.