In short
IV nutrient therapy is the intravenous administration of vitamins, minerals, and other nutrients directly into the bloodstream, bypassing the digestive system's absorption limits, most commonly given as variations of the "Myers' cocktail" (a mix including B vitamins, vitamin C, and magnesium).
Bypassing the gut allows a higher and more immediately available blood concentration of a nutrient than oral intake typically achieves, since oral absorption is limited by gut transporter capacity, and this is the legitimate clinical basis for IV nutrient use in specific, well-defined situations: severe nutrient deficiencies with malabsorption, certain conditions requiring high-dose vitamin administration under medical supervision, and standard hospital IV fluid and electrolyte replacement.
The wellness-market IV therapy offered at many longevity and infusion clinics — generally marketed for energy, immune support, hangover relief, athletic recovery, or anti-aging — is administered to people without a diagnosed deficiency or malabsorption condition, and this is where the evidence gap is largest: for someone who is not deficient, the body's kidneys generally excrete excess water-soluble vitamins (like vitamin C and B vitamins) fairly efficiently, and there is limited to no controlled human trial evidence that these wellness-context IV protocols produce meaningful, durable benefits beyond an oral supplement, for someone with normal nutrient status.
IV administration also carries its own risks not present with oral supplementation, including infection at the injection site, vein irritation, and — because it bypasses normal absorption regulation — a higher risk of overdosing certain nutrients (fat-soluble vitamins in particular, which the body doesn't excrete as readily as water-soluble ones) compared to oral intake.