It doesn't take 18 years to develop tolerance, most likely you are just getting older so your body can't recover from the drugs quickly enough any longer.

I've been on it for four years.

Many times I've thought that I've built up a tolerance, but in every scenario, it turns out that I was getting close to malnutrition or dehydration.

Try drinking much more water throughout the day. Also try eating a lot of carbs each night. These two things have made a tremendous difference for me.

Myself and a few others I know take Vyvanse instead of Ritalin or Adderall. I haven't had built any resistance yet but it's only been a year or two.

I like it better as it feels less harsh on my system. My doctor said that people will metabolize each type of ADHD medicine differently. Maybe a different compound would work better for you.

Not sure...so just throwing out ideas here. Perhaps try meditation for 20 minutes or so a day and twice daily exercise to help too. Maybe go off the Adderall for a week or two...try to step up the meditation/exercise (maybe coffee too)...then try the Adderall again? Or take it less frequently, like 4 days a week?

One other thought. Check the manufacturer of your Adderall. The "potency" can vary significantly between different manufacturers.

This exactly. I had to switch pharmacies from Walgreens to another local chain because of this. All of a sudden it seemed like the medication wasn’t working at all. One day I was out and had to use my “backup” meds which were a few months old and I noticed they looked slightly different. Taking them was like the first time I ever took Adderall, literally felt like my heart was going to explode.

I’d suggest filling your next prescription somewhere else and see if that makes a difference. Not just a different location, but a different pharmacy entirely.