I don't know about ADHD, but for anyone considering experimenting here, please, please, PLEASE be aware of the phenomenon called "Niacin Flush".

https://www.medicalnewstoday.com/articles/niacin-flush

"Many people taking niacin may experience a niacin flush. People may experience flushing around 30 minutes after taking a high dose of niacin, such as 500 milligrams (mg) or above.

Symptoms of niacin flush may include:

the skin on the face and upper body flushing

itching, tingling, or prickly sensations on the skin

a hot or burning sensation on the skin

Symptoms of niacin flush may clear within 1 to 2 hours of taking niacin.

People may find that symptoms of niacin flush are most intense when they first start taking high doses of niacin but may reduce with continued use.

According to the NIH, niacin supplements of 30 mg or more may cause headaches, rashes, or dizziness.

Taking high doses of niacin of 1,000 mg or more without the approval of a doctor may have harmful side effects, such as:

low blood pressure, which may increase the risk of falling

fatigue

high blood sugar levels

nausea

heartburn

abdominal pain

blurred vision and buildup of fluid in the eyes"

I usually feel a bit better with extra B-complex and have a family member who went carnivore and swears their symptoms improve (reading between the lines, more b vitamin. Don’t do carnivore. It’s gross and unhealthy)

A reasonable supplement might help. Past a certain point, your body just excretes it, so unless you’ve got kidney issues it’s pretty safe to try (eta at reasonable dosages. If you’re like the other guy and the army gave you so much you’re sweating it out, you’ve gone too far). You just make urine more expensive as the dose increases.