They are effective at first, and then you become tolerant and either need to increase your dosage for the same effect, or take a break (which can be difficult to suffer through because now you're operating at less than baseline functioning until homeostasis kicks in, which can take a week or longer). And e.g. Adderall was only released in 1996, so there hasn't been much time to study the long-term effects.

This happens with most drugs, not just for ADHD. Even caffeine (another kind of CNS stimulant, but less potent). Remember your first cup of coffee, how nice and awake you felt after you drank it?

Please don't take this the wrong way, btw. This is coming from someone who was also diagnosed with ADHD but stopped taking the meds because I didn't think they were worth it in the long term. I just drink coffee and take occasional week-long coffee breaks now. But everyone's different and I'm sure these drugs are an absolute necessity for some people, despite the risks.