Modafinil: general

Modafinil is a wakefulness-promoting drug (a eugeroic), not an amphetamine, though it acts on similar systems [1][2][3]. It's approved in the US for narcolepsy, obstructive sleep apnea, and shift work sleep disorder, and it's prescribed off-label for other things too [1][4]. In vitro work shows it blocks dopamine reuptake and raises extracellular dopamine, and it also activates glutamate signalling while inhibiting GABA, but its exact mechanism isn't fully worked out [5]. It's also been used outside medical settings as a so-called "smart drug" for its reported cognitive effects [2].

The standard prescribed dose is 200 mg once a day [3]. It's been studied for safety in over 3,500 patients [3]. That doesn't establish that the drug is risk-free at that dose or any other — it's simply the figure that exists.

Modafinil inhibits two liver enzymes, CYP3A4 and CYP2C19 [6]. Enzyme inhibition can change how other drugs are processed by the body; if you're taking anything else, prescribed or not, treat the combination as unpredictable rather than assumed safe.

In some places modafinil is a controlled substance because it's considered to have addiction potential [2]. Its legal status varies by jurisdiction — check the legal status page for specifics rather than assuming.

As with stimulants generally, effects of an overdose vary by the person, the dose, and how it's taken, and not every overdose looks the same [7]. If you're going to use modafinil outside medical supervision: don't use it alone, don't mix it with alcohol or other stimulants, stay hydrated, and get help fast for chest pain, seizure, or extreme agitation.

References

1. Modafinil - StatPearls - NCBI Bookshelf — NCBI Bookshelf (National Library of Medicine)
2. modafinil | Ligand page | IUPHAR/BPS Guide to PHARMACOLOGY — IUPHAR/BPS Guide to PHARMACOLOGY
3. Label: MODAFINIL tablet — NIH DailyMed
4. Modafinil: MedlinePlus Drug Information — MedlinePlus (National Library of Medicine)
5. Modafinil: Uses, Interactions, Mechanism of Action | DrugBank Online — DrugBank
6. Modafinil | C15H15NO2S | CID 4236 - PubChem — NIH/National Library of Medicine
7. Stimulant Guide: Overdose Prevention and Response — Centers for Disease Control and Prevention