What bioavailability means for supplements, and how much of a dose reaches you

Reviewed by Dr. Tracy Gibbs, PhD, Head of R&D · Last updated: 2026-10

Bioavailability is the share of a dose that reaches your circulation, and for some supplements it is far smaller than the label suggests. In a pharmacokinetic study, swallowed melatonin tablets of 2 mg and 4 mg showed an absolute bioavailability of approximately 15% (DeMuro et al., 2000), which means a 2 mg tablet put roughly 0.3 mg into the bloodstream.

The milligrams printed on a box describe what went into the product. They say nothing about what arrives. That gap depends on two things, the molecule and the route it takes, and it is the starting point for the way Smart Strips are designed.

What is bioavailability, in plain terms?

Bioavailability is the fraction of a dose that enters general circulation unchanged, ready to travel to the tissues where it acts. It is expressed as a percentage. Whatever is lost before that point, in the stomach, in the wall of the intestine or in the liver, does not count.

The percentage belongs to a molecule taken by a particular route, and two ingredients in the same capsule can behave in completely different ways. Caffeine swallowed in solution showed an absolute bioavailability of 108.3 ± 3.6% (Blanchard and Sawers, 1983). A value slightly above 100 reflects measurement variation; in practice, caffeine is absorbed essentially in full. Melatonin sits at the other end. A systematic review of its pharmacokinetics found that the bioavailability of oral melatonin ranged from 9 to 33% (Harpsøe et al., 2015).

How is bioavailability measured?

Researchers give a dose, take blood samples at fixed intervals and plot the concentration over time. The curve rises, peaks and falls as the body clears the substance. Three numbers describe it:

• Cmax is the peak concentration, the highest point of the curve.
• Tmax is the time it takes to reach that peak. It is the closest thing pharmacology has to a measure of speed.
• AUC, the area under the curve, adds up the concentration over the whole time the substance is present. It measures total exposure: how much actually arrived.

Caffeine in solution reached its peak after 29.8 minutes on average (Blanchard and Sawers, 1983). Immediate-release melatonin taken by mouth reaches its peak in approximately 50 minutes and has a half-life of about 45 minutes (Harpsøe et al., 2015), so the hour in which you take it shapes the result as much as the dose. Timing is a subject of its own, covered in when to take supplements.

What is the difference between absolute and relative bioavailability?

Absolute bioavailability compares a dose taken by mouth with the same dose given into a vein. An intravenous dose puts 100% of the substance into circulation by definition, so it serves as the reference. Divide the AUC after the oral dose by the AUC after the intravenous one, and you have the absolute figure. The melatonin and caffeine numbers above were both measured this way.

Relative bioavailability compares two forms, neither of them intravenous: a gum against a capsule, a spray against a tablet, a film against a tablet. It shows whether one form delivers more or less than the other, without saying how much of either reaches circulation in absolute terms. Most comparisons between supplement formats are relative, and many are never measured at all.

The distinction protects you from a common misreading. "Twice the bioavailability of a capsule" can describe a large difference or a trivial one, depending on where the capsule started.

Why does so much of a swallowed dose never arrive?

A swallowed dose passes three checkpoints before it reaches general circulation: the stomach, the intestinal wall and the liver. Everything absorbed from the gut travels first to the liver through the portal vein, and the liver breaks down a share of it. This is first-pass metabolism, and its size depends entirely on the molecule.

Some molecules barely survive the trip. A review of oral delivery of proteins and peptides notes that the absolute oral bioavailability of most of them is less than 1% (Shaji and Patole, 2008), because stomach acid and digestive enzymes break them down first. Live bacteria face a similar wall: a review of probiotic transit describes the acidic environment of the stomach as lethal to most bacteria, especially those that are not acid resistant (Han et al., 2021). In medicines, asenapine shows how much the route alone can change: its absolute bioavailability is 35% when it dissolves under the tongue and below 2% when it is swallowed (SAPHRIS prescribing information). Melatonin, with its 9 to 33% range, loses most of a swallowed dose. Caffeine passes nearly untouched.

Two capsules with identical milligrams can therefore deliver very different amounts, and the box gives you no way of telling which is which. The other reasons a supplement disappoints are covered in why your supplements aren't working.

Is faster absorption the same as more absorption?

No. Speed and amount are measured separately, and a format can win on one while losing on the other. Caffeine gives the clearest example. In a study of 84 healthy men, caffeine in chewing gum peaked earlier than caffeine in capsules, with a mean Tmax of 44.2 to 80.4 minutes against 84.0 to 120.0 minutes. Its relative bioavailability, however, was 64%, 74% and 77% of the capsule at doses of 50, 100 and 200 mg (Kamimori et al., 2002). The gum was quicker and delivered less.

Keep that in mind when you read the word "fast" on any pack, ours included. The gut already absorbs almost all of a caffeine dose, so the case for the 20 mg in FCKN FIT, taken 15 minutes before training, is timing and convenience: no shaker, no water.

The opposite pattern also exists: the same amount at the same speed. When oral films of several medicines were tested against the matching tablet, they delivered the same amount of active ingredient. In one study the concentration curves of film and tablet were "nearly superimposable" (Radicioni et al., 2017); in another, film and tablet shared a median Tmax of 2.5 hours (Park et al., 2018). With vitamin D, an oral spray raised blood levels about as much as a capsule, with no significant difference between them (Todd et al., 2016).

A film, a spray or a gum is a container. It changes the outcome where part of the dose is absorbed through the lining of the mouth and the swallowed route loses a great deal. For a molecule the gut already absorbs well, the gains are of a different kind: no water, nothing to swallow, and a dose taken at the moment it is meant for.

Can food change the bioavailability of a supplement?

Yes, for some ingredients. Fat-soluble vitamins need fat to be absorbed well. In a study of 50 healthy older adults given a single 50,000 IU dose of vitamin D3 with a test breakfast, peak blood vitamin D3 was 32% higher when the meal contained fat than when it was fat-free (Dawson-Hughes et al., 2015). Vitamin E behaves the same way: in a small crossover study, absorption was significantly greater with a toast and butter breakfast containing 17.5 g of fat, and low after a low-fat meal or water alone (Jeanes et al., 2004).

Both studies used doses far larger than a daily supplement, so they show a principle rather than a number for any product: the same capsule can behave differently on a morning with eggs and on a morning with black coffee.

Two strips in our range carry fat-soluble vitamins. IMMUNE contains vitamin D3 35 µg (700% NRV), vitamin C 20 mg (25% NRV) and elderberry juice concentrate 20 mg; it is a morning habit rather than something you feel. Vitamin D contributes to the normal function of the immune system, and vitamin C contributes to the normal function of the immune system. BEAUTÉ combines hydrolysed marine collagen 20 mg, hyaluronic acid 10 mg, vitamin E 3 mg (25% NRV), vitamin A 1000 µg (125% NRV) and biotin 100 µg (200% NRV). Vitamin E contributes to the protection of cells from oxidative stress. The part of each strip that is swallowed with saliva follows the ordinary digestive route, so it meets whatever breakfast you have had, in the same way a capsule would.

Does the label dose match what is in the product?

Not always, and bioavailability starts from the real content, not the printed one. An independent laboratory analysis of 25 melatonin gummy products sold in the US found that the actual melatonin content ranged from 74% to 347% of the labelled amount, and 22 of the 25 were inaccurately labelled (Cohen et al., 2023). On those figures, a gummy sold as 1 mg could hold about a quarter less, or more than three times as much.

For vitamins and minerals, EU guidance sets tolerances for food supplements: measured vitamin content may sit between 20% below and 50% above the declared value, and it must stay within that range for the entire shelf life (European Commission, 2012). The guidance sets no such tolerance for non-nutrients such as melatonin or botanical extracts. For the format question around melatonin specifically, see melatonin strips vs melatonin gummies.

What does bioavailability mean for an oral dissolvable strip?

An oral dissolvable strip changes the route for part of the dose. The strip dissolves on the tongue in 7 to 30 seconds. Part of what it carries is absorbed through the lining of the mouth, and that part avoids first-pass liver metabolism; the rest is swallowed with saliva and follows the same path as a capsule. As a format, compared with a tablet, Smart Strips is built for up to 95% bioavailability, up to 4x faster absorption and up to 30% higher effectiveness. How much any single ingredient gains depends on the molecule, which is what the research above shows.

That logic runs through the range:

• Z-z-Zen combines saffron extract 14 mg, L-glycine 12 mg, L-theanine 12 mg and melatonin 1 mg, taken 30 to 60 minutes before sleep, for the nights when you want to fall asleep more easily and stay asleep until morning. Melatonin is not the main ingredient, but it is the one whose swallowed route loses the most. The 1 mg matches the EU authorised claim: "Melatonin contributes to the reduction of time taken to fall asleep. The beneficial effect is obtained by consuming 1 mg of melatonin close to bedtime."
• FOCUS carries caffeine 20 mg, alpha-GPC 20 mg, L-theanine 10 mg, citicoline 10 mg and phosphatidylserine 5 mg, taken 15 to 20 minutes before work that needs attention. As with FCKN FIT, the gain is the moment, taken at your desk with nothing to drink, rather than the milligrams.

The mechanism is covered in depth in sublingual vs swallowed supplements and on the Science page. The way the strip itself is made is explained in QuickRelease™ oral strip technology.

If you are pregnant, nursing, taking medication or have a medical condition, consult your doctor before using any supplement.

Read the next label you pick up with one question in mind: how much of this reaches me, and by which route. Then explore the Smart Strips range, starting with Z-z-Zen for the evening.

FAQ

What is supplement bioavailability?

It is the share of a dose that reaches general circulation unchanged. It depends on the molecule and the route: swallowed caffeine is absorbed essentially in full (108.3%, Blanchard and Sawers 1983), while swallowed melatonin tablets reach about 15% (DeMuro 2000).

What is the difference between absolute and relative bioavailability?

Absolute bioavailability compares an oral dose with the same dose given intravenously, which counts as 100%. Relative bioavailability compares two non-intravenous forms, such as a gum and a capsule.

What do Cmax, Tmax and AUC mean?

Cmax is the peak blood concentration, Tmax is the time taken to reach it, and AUC, the area under the concentration curve, is the total exposure over time.

What is first-pass metabolism?

It is the breakdown a swallowed substance undergoes in the gut wall and the liver before it reaches general circulation. In medicines, asenapine reaches 35% when dissolved under the tongue and below 2% when swallowed.

Does faster absorption mean more of a supplement is absorbed?

Not necessarily. Caffeine gum peaked earlier than capsules but delivered 64 to 77% of the capsule's exposure (Kamimori 2002).

Is the dose on a supplement label always accurate?

Not always. In 25 melatonin gummy products sold in the US, actual content ranged from 74% to 347% of the label (Cohen 2023).

Do fat-soluble vitamins need to be taken with food?

They are absorbed better with a meal that contains fat. In one study, peak vitamin D3 was 32% higher with a fat-containing breakfast than with a fat-free one (Dawson-Hughes 2015).

Sources

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2. Harpsøe NG, Andersen LPH, Gögenur I, Rosenberg J. Clinical pharmacokinetics of melatonin: a systematic review. Eur J Clin Pharmacol. 2015;71(8):901-909. https://doi.org/10.1007/s00228-015-1873-4
3. Blanchard J, Sawers SJA. The absolute bioavailability of caffeine in man. Eur J Clin Pharmacol. 1983;24(1):93-98. https://doi.org/10.1007/BF00613933
4. Shaji J, Patole V. Protein and peptide drug delivery: oral approaches. Indian J Pharm Sci. 2008;70(3):269-277. https://doi.org/10.4103/0250-474X.42967
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8. Radicioni M, Castiglioni C, Giori A, Cupone I, Frangione V, Rovati S. Bioequivalence study of a new sildenafil 100 mg orodispersible film compared to the conventional film-coated 100 mg tablet administered to healthy male volunteers. Drug Des Devel Ther. 2017;11:1183-1192. https://doi.org/10.2147/DDDT.S124034
9. Park S-I, Heo S-H, Kim G, et al. Comparison of tadalafil pharmacokinetics after administration of a new orodispersible film versus a film-coated tablet. Drug Des Devel Ther. 2018;12:935-942. https://doi.org/10.2147/DDDT.S155040
10. Todd JJ, McSorley EM, Pourshahidi LK, Madigan SM, Laird E, Healy M, Magee PJ. Vitamin D3 supplementation in healthy adults: a comparison between capsule and oral spray solution as a method of delivery in a wintertime, randomised, open-label, cross-over study. Br J Nutr. 2016;116(8):1402-1408. https://doi.org/10.1017/S0007114516003470
11. Dawson-Hughes B, Harris SS, Lichtenstein AH, Dolnikowski G, Palermo NJ, Rasmussen H. Dietary fat increases vitamin D-3 absorption. J Acad Nutr Diet. 2015;115(2):225-230. https://doi.org/10.1016/j.jand.2014.09.014
12. Jeanes YM, Hall WL, Ellard S, Lee E, Lodge JK. The absorption of vitamin E is influenced by the amount of fat in a meal and the food matrix. Br J Nutr. 2004;92(4):575-579. https://doi.org/10.1079/BJN20041249
13. Cohen PA, Avula B, Wang YH, Katragunta K, Khan I. Quantity of melatonin and CBD in melatonin gummies sold in the US. JAMA. 2023;329(16):1401-1402. https://doi.org/10.1001/jama.2023.2296
14. European Commission, DG SANCO. Guidance document for competent authorities for the control of compliance with EU legislation with regard to the setting of tolerances for nutrient values declared on a label. December 2012. https://assets.publishing.service.gov.uk/media/5a7cc3eced915d63cc65cb79/EU-Guidance-on-Tolerance.pdf
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