Pharmacokinetics
How a drug moves through the body determines what it does. Same molecule, same milligrams, very different outcomes depending on route and timing.
3.1 Bioavailability (F): How Much Actually Arrives
Bioavailability = the percentage of a dose that reaches the bloodstream. Depends on the route.
- Oral — lowest. Drug passes through the liver before reaching the brain ("first-pass metabolism"). The liver destroys a major fraction.
- Snorted / under the tongue — largely bypasses the liver (some first-pass metabolism still occurs depending on drug and dose). Faster, more potent.
- Smoked / vaped — lungs to brain in seconds.
- Intravenous (IV) — 100%. No filters.
The dose to the brain can be 5–10× higher by a different route.
The speed of onset is one of the main indicators of addiction potential. Smoking something will be more addictive than eating it.
3.2 The Blood-Brain Barrier
The brain is sealed off from the bloodstream by a tight wall of cells. Only certain molecules pass.
- Fat-soluble (lipophilic) drugs — cross instantly.
- Water-soluble drugs — slow or blocked.
One key difference between meth and amphetamine (a key active ingredient in Adderall, which contains mixed amphetamine salts in a 75:25 dextro:levo ratio) is lipophilicity. Meth is more lipophilic — it crosses the blood-brain barrier more easily and reaches the brain faster, contributing to its sharper subjective onset.
3.3 Half-Life vs. Active Duration
- Active duration — how long the drug is actively having an effect. The "high."
- Half-life — how long it takes the liver to remove 50% of the drug from the entire body. Most hepatic drug metabolism is carried out by CYP450 enzymes (cytochrome P450 — a family of liver enzymes). Individual variation in which CYP450 variants someone carries determines how fast or slow they metabolize a given drug. Two people taking the same dose can have meaningfully different blood concentrations.
The high ends when the drug unbinds and redistributes into fat or blood or when the body adapts in real time.
The drug is still in the body for hours after the felt effect ends.
