You can take the world's highest-dose vitamin supplement and still be deficient if your body can't absorb it. The science of bioavailability explains why injections are not just a more expensive option — they are a fundamentally different delivery method that changes what your cells actually receive.
Bioavailability is the proportion of a nutrient that, once taken, actually reaches the bloodstream and is available for use by cells and tissues. A vitamin with 100% bioavailability delivers every milligram you take to your body. A vitamin with 5% bioavailability delivers one twentieth — the rest is excreted or broken down before it reaches the bloodstream.
Oral supplements — tablets, capsules, sprays, gummies — are all subject to the same fundamental barrier: the gastrointestinal tract. Before any nutrient reaches the blood, it must survive stomach acid, survive digestive enzymes, pass through the intestinal wall, and avoid first-pass metabolism in the liver. Each of these steps reduces the amount that ultimately reaches your cells.
Injectable vitamins bypass every one of these steps. An intramuscular (IM) or intravenous (IV) injection delivers the nutrient directly into the bloodstream or muscle tissue. Bioavailability is effectively 100%.
| Nutrient | Oral Absorption Rate | IM / IV Absorption | Lambert Price (IM) |
|---|---|---|---|
| Vitamin B12 | 1–5% | ~100% | £40 |
| Vitamin D | Variable (15–40%) | ~100% | £50 |
| Glutathione | <1% (broken down) | ~100% | £80 |
| NAD+ (NMN/NR oral) | ~10–25% | ~100% | £60 (IM) / £150+ (IV) |
| Biotin (B7) | ~30–40% (variable) | ~100% | Call for price |
| Vitamin C (high dose) | <20% above 200mg | ~100% | In IV drips |
Vitamin B12 is the most striking example of the oral absorption problem. The body absorbs B12 through a complex process that requires a protein called intrinsic factor, produced by stomach cells. At a standard oral dose, only 1–5% of B12 is typically absorbed — meaning a 1,000 mcg tablet delivers between 10 and 50 mcg to the bloodstream.
Absorption worsens significantly with age (stomach cells produce less intrinsic factor), with certain medications (metformin, PPIs), with stomach conditions, and in vegans and vegetarians whose baseline B12 levels may already be low. A high-street 1,000 mcg B12 tablet may be essentially useless for an older patient on omeprazole with no intrinsic factor production.
An intramuscular B12 injection (£40 at Lambert) delivers the full dose directly into the muscle, from where it passes into the bloodstream at near-100% absorption. The clinical result is a rapid, reliable rise in serum B12 — measurable within days. Compare this to months of daily tablets achieving uncertain results.
Glutathione is perhaps the most extreme case. It is a tripeptide — three amino acids bonded together. When taken orally, the digestive enzymes in the gut break these bonds as a matter of routine, reducing glutathione to its three constituent amino acids (cysteine, glycine, glutamine) before it ever reaches the bloodstream. The original glutathione molecule is destroyed before absorption can occur.
Some products claim to use "liposomal glutathione" — encapsulated in a fat bubble to survive digestion — with better results, but even these deliver a fraction of the therapeutic dose compared to an IM injection. For any meaningful clinical benefit from glutathione, injection is the only reliable route.
Vitamin D is a fat-soluble vitamin, meaning it requires dietary fat for absorption. Take a Vitamin D tablet on an empty stomach, or with a low-fat meal, and absorption drops substantially. For older adults with declining gut function, people with malabsorption conditions (coeliac, Crohn's, chronic pancreatitis), or anyone who needs rapid correction of established deficiency, tablets are often too slow and unreliable.
A high-dose IM Vitamin D injection (£50 at Lambert) bypasses all these limitations. Levels typically rise significantly within days, with the effect lasting 3–6 months. For older adults — where Public Health England recommends daily supplementation but the NHS will not prescribe it — a twice-yearly private injection is often the most clinically effective and practically convenient solution. Read our full guide for over 65s.
NAD+ itself cannot be taken orally — it does not survive digestion in its intact form. Oral NAD+ supplements use precursors: NMN (nicotinamide mononucleotide) or NR (nicotinamide riboside), which are converted to NAD+ in cells. These precursors have better oral bioavailability than NAD+ itself, but still face absorption and conversion limitations — only 10–25% reaches meaningful cellular concentrations.
An NAD+ IM injection (£60 at Lambert) delivers NAD+ directly. An NAD+ IV infusion (from £150) delivers it straight into the bloodstream for immediate cellular uptake. For those seeking genuine, measurable effects on energy, cognitive function, and cellular health — rather than the marginal effects of oral supplements — direct delivery makes a significant clinical difference.
To be balanced: oral supplementation works well for many people in many circumstances. Someone with good gut health, no absorption issues, and mild insufficiency who consistently takes a daily supplement will likely benefit from tablets. They are convenient, inexpensive, and available without a clinical appointment.
Injections become clearly superior when: