Intravenous

Intravenous administration delivers a substance directly into a vein, placing it into systemic circulation without any absorption step. Nothing crosses a membrane or passes through the gut — the substance is already in the bloodstream, carried immediately to the heart and from there distributed to every tissue in the body.

Clinically abbreviated IV; in common usage the route is described as injecting. It requires a needle and syringe; in medical settings it may be delivered through a catheter as a continuous infusion rather than a single bolus.

Onset & absorption

Because there is no absorption step, onset is the fastest of any common route. Effects can begin within seconds of injection, and the rise is abrupt — not the gradual curve of oral dosing or the moderate slope of insufflation. Blood concentration peaks almost immediately.

First-pass metabolism — the liver's modification of a substance before it reaches systemic circulation — does not apply here. A substance taken orally is substantially processed by the gut wall and liver before reaching the brain; the same compound given intravenously arrives largely unmodified. This can make the experience qualitatively as well as quantitatively different from oral administration.

The steep, compressed rise leaves almost no window between injection and full effect.

Harm reduction

The risks of intravenous administration are inherent to the route itself, independent of the substance.

Repeated injections damage veins over time through inflammation and scarring, eventually collapsing them. Infection risk is high: bacteria introduced through the skin or carried on equipment reach the bloodstream directly and can cause septicaemia or endocarditis, a serious infection of the heart's inner lining. Sharing needles or other injection equipment transmits bloodborne pathogens including hepatitis B, hepatitis C, and HIV.

Particulate matter in a substance not formulated for injection can occlude small vessels. Contaminants that would be filtered or neutralised through other routes arrive in circulation unchanged.

The most consequential route property is irreversibility. Oral absorption allows a retrieval window — activated charcoal or gastric emptying can still limit exposure. Once a substance is in the vein, that window is closed. Dosing errors produce their full effect before any intervention is possible.

What varies by substance

Intravenous administration determines how a substance arrives, not what it does. Duration, potency, organ toxicity, the character of the effect, and every other pharmacological property belong to the substance record.

How much the IV route changes the experience relative to other routes varies considerably by compound. Where first-pass metabolism substantially modifies an oral dose, bypassing it matters more; where another route already delivers high systemic exposure, the change may be smaller.

Substances formulated for clinical IV use are prepared at appropriate pH, free of particulates, and dosed for direct injection. Substances not formulated this way carry hazards beyond their pharmacology — hazards that are route-specific and addressed on the substance record, not here.