Ibuprofen: the pathway in the body

Ibuprofen is part of the pathway “Ibuprofen”. This page shows the whole pathway; the station of Ibuprofen is highlighted.

Where this laboratory value sits: Ibuprofen — mixture of two mirror forms. Ibuprofen is swallowed as a tablet. It is present as a mixture of two mirror-image forms, R and S. Only the S form fits into the channel of cyclooxygenase. The R form serves as a store for the S form. Source 1, 11

In brief

Ibuprofen belongs to the non-steroidal anti-inflammatory agents. It lays itself into the channel of cyclooxygenase through which arachidonic acid travels to the active site. The channel is occupied while the substance is there — not destroyed. Fewer prostaglandins then form.

The pathway step by step

Each station states what the compound does there. Three signs: ↑ supplies — builds up or makes available · ↓ depletes — inhibits, consumes or withholds · ↕ both, depending on amount. Behind it stands what the statement rests on: established physiology, observed in studies, or contested. The signs do not grade; they name the direction.

1. Ibuprofen → Uptake in the gut The substance is absorbed rapidly; the label puts the highest level in the blood at one to two hours after a dose. In the blood it is bound for the most part to albumin. Only the free part reaches the tissue. Source 1, 11↑ supplies The blood distributes the substance through the body. Binding to albumin holds a store; only the free part passes into the tissue and reaches cyclooxygenase. established physiology Source 1
   ⚖ When the balance tips

   too much — If there is a lot of ibuprofen in the blood, the binding sites on albumin become scarce; the free part then grows faster than the total amount, and more of it reaches tissue and liver.

   too little — If there is little ibuprofen in the blood, nearly all of it is bound to albumin; only a small free part passes into the tissue.

   established physiology · Source 1

2. Uptake in the gut → S-ibuprofen isomerase (R to S) An enzyme path of its own converts part of the R form into the S form; the other direction does not happen. The share of the form that acts at the enzyme thereby rises. The inhibition is reversible. Source 1, 2↓ depletes S-ibuprofen occupies the channel of cyclooxygenase and keeps arachidonic acid away from the active site. The inhibition is reversible: when the level falls, it leaves the enzyme again. established physiology Source 2, 3
   ⚖ When the balance tips

   too much — If a lot of S-ibuprofen is present, the channels of both COX forms are largely occupied; in many tissues little PGH₂ then forms, and so little prostaglandin and thromboxane.

   too little — If little S-ibuprofen is present, arachidonic acid displaces it from the channel more easily, because both compete for the same place; prostaglandin formation largely continues.

   established physiology · Source 2, 3

3. S-ibuprofen → Breakdown in the liver CYP2C9 In the liver CYP2C9 above all hangs oxygen onto the side chain. The fragments are coupled and leave with the urine; the label calls the excretion complete one day after the last dose. The fragments are inactive. Source 1, 11↓ depletes CYP2C9 turns ibuprofen into hydroxy and carboxy compounds that no longer act at the enzyme. Breakdown in the liver thus determines how long the channel of cyclooxygenase stays occupied. established physiology Source 1
   ⚖ When the balance tips

   too much — If CYP2C9 works quickly, the level falls sooner, and the channels of cyclooxygenase become free again earlier.

   too little — If CYP2C9 works more slowly, S-ibuprofen stays in the blood longer and occupies the enzyme for longer.

   established physiology · Source 1

4. In the cell membrane → Free arachidonic acid cPLA2 · calcium When calcium in the cell rises, the phospholipase cPLA2 travels to the membrane and cuts arachidonic acid out. Only as a free molecule is it converted further. Without free fatty acid, cyclooxygenase rests. Source 4↑ supplies Free arachidonic acid is the starting material that cyclooxygenase needs. Without a stimulus and calcium hardly any is released, and the path rests. established physiology Source 4, 3
   ⚖ When the balance tips

   too much — If a lot of arachidonic acid is released, the cyclooxygenases convert more of it; if S-ibuprofen occupies the channel, the fatty acid competes with it for the same place.

   too little — If little arachidonic acid is released, cyclooxygenase has no starting material, and hardly any prostaglandins form, even when the enzyme is free.

   established physiology · Source 3, 4

5. Free arachidonic acid → PGH₂ COX-1 and COX-2 · haem iron, oxygen Cyclooxygenase builds in oxygen and forms a ring; PGG₂ arises. The peroxidase part of the same enzyme, a haem with iron, turns that into PGH₂ — the common precursor of the prostaglandins and of thromboxane. The cell decides the end product. Source 3↑ supplies PGH₂ is short-lived and is passed on at once: which synthase a cell carries decides whether it becomes prostaglandin E₂, thromboxane A₂ or another prostanoid. established physiology Source 3, 7
   ⚖ When the balance tips

   too much — If a lot of PGH₂ forms, the downstream synthases convert more of it; where COX-2 and mPGES-1 are made together, as in inflamed tissue, mainly prostaglandin E₂ forms.

   too little — If little PGH₂ forms, all downstream synthases receive less starting material; prostaglandins and thromboxane decrease together.

   established physiology · Source 3, 7

6. PGH₂ → Prostaglandin E₂ PGE synthases · glutathione Tissue-specific synthases make the individual prostaglandins out of PGH₂. Prostaglandin E₂ arises in the stomach wall continuously and is at the same time among the substances with which the body sets an inflammatory reaction going. It acts via EP receptors. Source 5, 7↑ supplies Prostaglandin E₂ acts via four EP receptors: in the stomach wall it maintains mucus, bicarbonate and blood flow; in inflamed tissue it widens vessels and makes nerve endings more sensitive. established physiology Source 7, 10
   ⚖ When the balance tips

   too much — If a lot of prostaglandin E₂ is present, as with an inflammatory stimulus, the vessels are wider, nerve endings respond to weaker stimuli, and in the brain the set point of body temperature is raised.

   too little — If little prostaglandin E₂ is present, the cells of the stomach wall release less mucus and bicarbonate, the lining receives less blood flow, and inflammatory signals become weaker.

   established physiology · Source 7, 5, 10

7. PGH₂ → Thromboxane A₂ thromboxane synthase In platelets, thromboxane synthase converts PGH₂ into thromboxane A₂. There this path runs through COX-1. It soon breaks down into inactive thromboxane B₂. Source 6↑ supplies Thromboxane A₂ is a short-lived messenger of the platelets: it makes further platelets attach and narrows vessels. After a short time it breaks down into inactive thromboxane B₂. established physiology Source 6
   ⚖ When the balance tips

   too much — If the platelets make a lot of thromboxane A₂, they draw in further platelets, and the vessels narrow more strongly at that site.

   too little — If the platelets make little thromboxane A₂, for instance because COX-1 is occupied, they stick together more slowly; with ibuprofen this lasts only while the substance sits at the enzyme.

   established physiology · Source 6

8. Prostaglandin E₂ → Mucus and bicarbonate EP receptors Prostaglandin E₂ binds to EP receptors of the gastric lining. The cells then release mucus and bicarbonate, and the blood flow of the lining increases. When COX-1 runs more slowly, less prostaglandin E₂ arises for this path. This neutralises acid. Source 5, 10↑ supplies Mucus and bicarbonate form a layer in which acid from the stomach interior is neutralised towards the wall; the blood flow carries away acid that still gets through. established physiology Source 5
   ⚖ When the balance tips

   too much — If the layer is thick and the lining well supplied with blood, a nearly neutral environment remains at the cell surface, even when there is a lot of acid in the stomach interior.

   too little — If the layer becomes thinner and the lining receives less blood flow, acid reaches the cells of the stomach wall more easily, and small damage is repaired more slowly.

   established physiology · Source 5

9. Thromboxane A₂ → Platelets TP receptors Thromboxane A₂ binds at the TP receptor of the platelets. They then change their shape and stick to one another. This forms a first plug at an injury. Source 6↑ supplies Adhering platelets form a first plug at an injured vessel wall; they release thromboxane A₂ themselves and so draw in further platelets. established physiology Source 6
   ⚖ When the balance tips

   too much — If adhesion is strongly stimulated, the plug grows quickly, and platelets also settle on altered sites of the vessel wall.

   too little — If the platelets stick together less strongly, a small injury to the vessel wall closes more slowly, and blood escapes for longer.

   established physiology · Source 6

Further stations

• Ibuprofen — mixture of two mirror forms Ibuprofen is swallowed as a tablet. It is present as a mixture of two mirror-image forms, R and S. Only the S form fits into the channel of cyclooxygenase. The R form serves as a store for the S form. Source 1, 11↑ supplies The mixture provides both mirror-image forms: the S form acts at the enzyme straight away, the R form serves as a store from which further S form arises over time. established physiology Source 1, 2
  ⚖ When the balance tips

  too much — If a lot of ibuprofen is present, more R form is also available; its conversion into the S form then keeps up the share of the active form for longer.

  too little — If little ibuprofen is present, the S form occupies only part of the cyclooxygenase channels; the remaining enzymes go on converting arachidonic acid.

  established physiology · Source 1, 2

• Cyclooxygenase — COX-1 and COX-2 Cyclooxygenase has a long channel through which the fatty acid travels to the active site. S-ibuprofen lays itself into this channel and blocks it while it sits there. Both forms of the enzyme are concerned. Less PGH₂ then forms. Source 2, 3↑ supplies Cyclooxygenase is the bottleneck of the path: only what it converts becomes prostaglandins and thromboxane. COX-1 works continuously in many tissues, COX-2 is made mainly in response to inflammatory stimuli. established physiology Source 3
  ⚖ When the balance tips

  too much — If COX activity is high, for instance when a lot of COX-2 is made in response to an inflammatory stimulus, plenty of PGH₂ forms and from it prostaglandin E₂.

  too little — If COX activity is low because the channel is occupied, less PGH₂ forms; this affects at the same time the path to prostaglandin E₂ in the stomach wall and to thromboxane in the platelets.

  established physiology · Source 3, 5, 6

• In the cell membrane — arachidonic acid built in Arachidonic acid sits for the most part in the phospholipids of the cell membranes, usually at the middle position of the glycerol backbone. There it stays until a stimulus reaches the cell. The membrane is its store. Source 4, 8↑ supplies The membrane is the store: only a stimulus makes arachidonic acid available to cyclooxygenase. It is formed from linoleic acid, or it comes directly from food. established physiology Source 4, 8
  ⚖ When the balance tips

  too much — If there is a lot of arachidonic acid in the membranes, a correspondingly large amount of starting material for series-2 prostaglandins and for thromboxane A₂ is available upon a stimulus.

  too little — If there is less arachidonic acid and more EPA in the membranes, less arachidonic acid is released upon a stimulus; cyclooxygenase then converts part of the EPA into series-3 prostaglandins.

  observed in studies · Source 9

What this active substance affects

• Arachidonic acid — Starting material of cyclooxygenase; by way of PGH₂ the whole series of prostaglandins and thromboxane arises from it Source 3, 4
• EPA (eicosapentaenoic acid) — The same cyclooxygenases also convert EPA; prostaglandins of series 3 arise from it; competes with arachidonic acid Source 9

What takes part in these steps

• Linoleic acid — Omega-6 fatty acid from food, from which arachidonic acid arises over several steps; stock for the membrane Source 8
• Calcium — When it rises in the cell, the phospholipase cPLA2 travels to the membrane and frees arachidonic acid; starts the path Source 4
• Iron — Sits as a haem in the peroxidase part of cyclooxygenase, which converts PGG₂ into PGH₂; without haem the enzyme rests Source 3
• Glutathione — Microsomal PGE synthase-1 needs glutathione in order to convert PGH₂ into prostaglandin E₂; without it, less PGE₂ forms Source 7

What the prescribing information states

The US prescribing information for the tablets lists the reactions that occurred in controlled trials in more than 1 of 100 participants; the column rests on about 3,000 participants. This label gives no placebo column alongside it — unlike, for instance, the ones for pantoprazole or ramipril. A row-by-row comparison is therefore not possible here.

How to read the table: because the placebo column is missing, no row shows how often the same report came in without the substance. The one comparison this label does draw it states in its text: at equally effective doses, complaints in the gastrointestinal area occurred about half as often as with acetylsalicylic acid or indometacin.

The prescribing information lists in addition reactions that occurred less often than in 1 of 100 participants, as well as reports received after approval. By its own account neither a frequency nor a causal relationship can be given for those; they are therefore not listed here.

Sources

1. Davies NM. Clinical pharmacokinetics of ibuprofen. The first 30 years. Clin Pharmacokinet 1998 · PubMed 9515184
2. Rainsford KD. Ibuprofen: pharmacology, efficacy and safety. Inflammopharmacology 2009 · PubMed 19949916
3. Smith WL, DeWitt DL, Garavito RM. Cyclooxygenases: structural, cellular, and molecular biology. Annu Rev Biochem 2000 · PubMed 10966456
4. Leslie CC. Regulation of the specific release of arachidonic acid by cytosolic phospholipase A2. Prostaglandins Leukot Essent Fatty Acids 2004 · PubMed 15041029
5. Wallace JL. Prostaglandins, NSAIDs, and gastric mucosal protection: why doesn't the stomach digest itself? Physiol Rev 2008 · PubMed 18923189
6. Mitchell JA, Kirkby NS, Ahmetaj-Shala B et al. Cyclooxygenases and the cardiovascular system. Pharmacol Ther 2021 · PubMed 32640277
7. Samuelsson B, Morgenstern R, Jakobsson PJ. Membrane prostaglandin E synthase-1: a novel therapeutic target. Pharmacol Rev 2007 · PubMed 17878511
8. Brenna JT, Kothapalli KSD. New understandings of the pathway of long-chain polyunsaturated fatty acid biosynthesis. Curr Opin Clin Nutr Metab Care 2022 · PubMed 34937850
9. Calder PC. Marine omega-3 fatty acids and inflammatory processes: Effects, mechanisms and clinical relevance. Biochim Biophys Acta 2015 · PubMed 25149823
10. Dey I, Lejeune M, Chadee K. Prostaglandin E2 receptor distribution and function in the gastrointestinal tract. Br J Pharmacol 2006 · PubMed 17016496
11. US prescribing information (United States): Ibuprofen Tablets USP, DailyMed, version of 15 Sept 2026, sections Description and Clinical Pharmacology · Prescribing information
12. US prescribing information (United States): Ibuprofen Tablets USP, DailyMed, version of 15 Sept 2026, sections Adverse Reactions and Postmarketing Experience · Prescribing information

Whole pathway: Ibuprofen

Related pathways

• Metamizol — in the cell membrane

As of 2026-09-25. Draft, written by Claude to schema v2; sources checked in PubMed; expert approval pending
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