Amphetamine: general

Amphetamine is a central nervous system stimulant [1][2] and the parent compound of a family of widely used and abused psychoactive drugs, including its own enantiomers levoamphetamine and dextroamphetamine [1][3]. It works by modulating dopamine, serotonin, and norepinephrine in the brain [4]. It also has real medical uses: it's a first-line treatment for ADHD in adults and children six and older, a second-line treatment for narcolepsy, and it's used as an appetite suppressant [2][5][4]. Lisdexamfetamine, a long-acting amphetamine medication, is FDA-approved for binge-eating disorder [2]. Excess norepinephrine and dopamine activity, a feature of stimulants including amphetamine, can produce symptoms of increased sympathetic nervous system activity: heightened alertness, dilated pupils, fast heart rate, high blood pressure, high body temperature, and seizures [6]. As with stimulant overdose generally, amphetamine overdose can lead to cardiovascular collapse, multiorgan failure, and death [6].

The two things to act on before anything else:

Overdose doesn't look one way. What counts as an overdose is simply effects severe enough to be life-threatening, and it varies with the person's health, the specific stimulant, how it's taken, and the dose [7]. Don't wait for a textbook picture — chest pain, seizure, or extreme agitation are reasons to get help, not reasons to wait and see [8].

Combinations are where a lot of the danger sits. As with stimulants generally, sympathomimetic toxicity can come from one drug taken to excess or from multiple drugs used in combination [6].

Beyond that: stay hydrated, and set up someone you can call if you start to feel panicked or paranoid [8].

References

1. Amphetamine - American Chemical Society — American Chemical Society
2. Amphetamine — NCBI Bookshelf (StatPearls)
3. Amphetamine — Wikipedia
4. Amphetamine and Its Salts — California Office of Environmental Health Hazard Assessment (OEHHA)
5. amphetamine | Ligand page | IUPHAR/BPS Guide to PHARMACOLOGY — IUPHAR/BPS Guide to Pharmacology
6. Sympathomimetic Toxicity – StatPearls — NCBI Bookshelf
7. Stimulant Guide: Overdose Prevention and Response — Centers for Disease Control and Prevention
8. Harm reduction and methamphetamine use — NSW Health