Ritalin is effective as long as the patient taking it uses it as prescribed by his/her doctor. It won't be effective if a patient doesn't take it.
Now, as for buying it online: Ritalin is a prescription drug only. Only a doctor can prescribe and, therefore, it won't be found online. It is illegal to sell prescription drugs online. Buying a prescription drug is not going to be the actual true drug. It will be a knock off drug and you would be wasting your money buying it.