Do you have to slowly wean a child off of 18 mg concerta or just stop abruptly?

No need to wean: There is no need to wean off Concerta 18 mg (or even higher doses).

Best to ask: It is best to ask your provider who provided it. 18mg of Concerta is already a low dose, so stopping abruptly is not that much of a drastic change. One option would be to give the medication every other day for 6 days: give, hold, give, hold, give, hold. And then stop. It is best to watch for any behavior change (for the better, or for the worse) when you make any change with medication.

Just stop: Not sure what the concern is, but 18 mg of Concerta is, as dr. Ahrendsen noted, the lowest dose available. It is quite common with all stimulants, even at much higher doses, to stop using the medication on weekends and long school holidays. The only "side effect" of stopping 18 mg of Concerta will be prompt return of any adhd symptoms it may have been suppressing. But work with your doctor!

Concerta: 18mg. is low dose & most times, if your doctor recommends discontinuation, it can be stopped.

Stop immediately: Concerta like other methylphenidate preparations and Adderall (dextroamphetamine and racemic amphetamine) only work for the day they are taken. They have been cleared by the next day. You can just not give the next dose. An example of this is some famililies do not give stimulants on weekends or holidays. There is no withdrawal response.