# Acidic Drugs: Absorption and Pharmacology Basics

A weak acid is a drug that carries a protonatable carboxyl, hydroxyl or sulfonamide group, so it exists in two interconverting forms: an uncharged, lipid-soluble acid (HA) and a negatively charged, water-soluble anion (A⁻). Absorption of drugs across the gut wall depends almost entirely on how much of that drug sits in the uncharged form at any given luminal pH, because only the uncharged form diffuses freely through the lipid bilayer of the enterocyte membrane.

This matters in every species a veterinarian treats. The same aspirin, furosemide, phenobarbital, ketoprofen or doxycycline tablet can be absorbed quickly, slowly, or barely at all depending on gastric pH, gastric emptying rate, food in the stomach, and the formulation it arrives in. Understanding pH partitioning turns a confusing list of "food effects" and "drug interactions" into one predictable physical rule.

This article is educational and is not a substitute for veterinary diagnosis or treatment.

## The Core Principle: pH Partitioning

The pH partition hypothesis states that a weak electrolyte crosses a biological membrane mainly in its unionized, lipid-soluble form, driven by the concentration gradient of that unionized species across the membrane [1]. The ionized form is too polar to dissolve in the hydrophobic core of the phospholipid bilayer, so it is effectively excluded from passive transmembrane diffusion.

Two consequences follow immediately.

1. The fraction of drug that is unionized at a given pH determines how much drug is available to diffuse.
2. Once unionized drug crosses into a compartment of different pH, it re-equilibrates and can become trapped as the ionized species, because the ionized form cannot easily diffuse back out. This is called ion trapping.

Ion trapping is not a gut-only phenomenon. It happens in lysosomes, in milk, in urine and in the renal tubule. A 2020 analysis of hydroxychloroquine and azithromycin calculated lysosomal drug concentrations exceeding cytosolic and extracellular concentrations by more than 50,000-fold, driven by exactly this mechanism [2]. The same physics that traps a weak base in an acidic lysosome traps a weak acid in an alkaline compartment.

### The Henderson-Hasselbalch Relation

The Henderson-Hasselbalch equation links pH, pKa and the ratio of ionized to unionized drug. For a weak acid:

**pH = pKa + log([A⁻] / [HA])**

Rearranged to solve for the unionized fraction:

**[HA] / ([HA] + [A⁻]) = 1 / (1 + 10^(pH - pKa))**

For a weak base the sign flips:

**[B] / ([B] + [BH⁺]) = 1 / (1 + 10^(pKa - pH))**

The pKa is the pH at which exactly half the drug is ionized and half is unionized. It is a fixed physicochemical property of the molecule, not something the body changes. Ketoprofen, for example, has a measured pKa of 4.76 [3]. Glibenclamide has a pKa of about 6.5 [4]. The drug L-870,810, a lipophilic weak acid studied in human intestinal aspirates, has a pKa of 7.3 [5].

### Worked Example: Aspirin in the Stomach and Intestine

Take a weak acid with a pKa of 3.5, the approximate value for acetylsalicylic acid. Compare two compartments: gastric fluid at pH 1.5 and proximal small intestinal fluid at pH 6.5.

**Stomach, pH 1.5:**

Unionized fraction = 1 / (1 + 10^(1.5 - 3.5)) = 1 / (1 + 10^(-2)) = 1 / 1.01 = **0.99**

About 99% of the drug is unionized and lipid-soluble. It is fully capable of diffusing across the gastric mucosa.

**Small intestine, pH 6.5:**

Unionized fraction = 1 / (1 + 10^(6.5 - 3.5)) = 1 / (1 + 1000) = 1 / 1001 = **0.001**

Only about 0.1% is unionized at any instant. Yet the small intestine absorbs the overwhelming majority of an oral aspirin dose. This is the central paradox of acidic drug absorption, and it is resolved by surface area and residence time, not by ionization alone.

## Why the Small Intestine Wins Despite Higher pH

The stomach has a small absorptive surface and a short residence time for most ingesta. The small intestine has a vastly larger surface, amplified by villi and microvilli, and a longer transit time. Even though only a tiny fraction of a weak acid is unionized at intestinal pH, the continuous removal of that unionized fraction by absorption pulls the equilibrium reaction (HA ⇌ H⁺ + A⁻) to the right, regenerating more unionized drug. Over a long segment with enormous surface area, that trickle becomes the dominant route of uptake.

This is not a theoretical claim. Glibenclamide, a weak acid with a pKa of about 6.5, was instilled directly into the stomach, duodenum and ascending colon of human subjects under visual control. The mean areas under the plasma concentration-time curve were 477 ± 131 ng·h/mL for the stomach, 475 ± 142 ng·h/mL for the duodenum, and 486 ± 301 ng·h/mL for the colon [4]. Total extent of absorption was essentially identical at all three sites. What differed was the rate. Absorption was faster from the duodenum than from the stomach, and slowest from the colon. The study authors explicitly discussed whether these findings confirm the pH partition hypothesis, and the answer is that they confirm it only when surface area and residence time are factored in alongside pH [4].

The practical lesson: ionization predicts *permeability*, but total absorption is permeability multiplied by surface area multiplied by contact time.

### The Unstirred Water Layer

A thin layer of unstirred fluid sits against the enterocyte brush border. Drug must diffuse across this layer before it can reach the membrane. Chemical reactions inside the unstirred layer, including the dissociation of weak acids, can dramatically affect net transport. Classic work with salicylate and salicylic acid across phospholipid bilayer membranes showed that adding appropriate buffering to salicylates can increase their rate of absorption from the stomach, because the buffer supplies protons that regenerate the unionized, permeant form right at the membrane surface [6]. The unstirred layer is not a passive gap. It is a reaction chamber.

## The Comparison Table: Weak Acids Versus Weak Bases

The table below contrasts the two classes at the two key gastrointestinal pH values. Values are illustrative for a weak acid with pKa 3.5 and a weak base with pKa 8.5, using the Henderson-Hasselbalch relation.

| Property | Weak acid (pKa 3.5) | Weak base (pKa 8.5) |
|--|--|--|
| Charge at low pH | Unionized (HA) | Ionized (BH⁺) |
| Charge at high pH | Ionized (A⁻) | Unionized (B) |
| Unionized fraction in stomach (pH 1.5) | ~99% | ~0.001% |
| Unionized fraction in intestine (pH 6.5) | ~0.1% | ~99% |
| Dominant absorption site | Small intestine | Small intestine (if it survives gastric acid) |
| Main absorption risk | Low gastric surface area, slow dissolution of weak acids with high pKa | Precipitation when gastric pH rises, food effects, acid-reducing drugs |
| Classic example | Aspirin, ketoprofen, glibenclamide | Posaconazole, dipyridamole, dasatinib |

The table makes an uncomfortable point. Weak bases are almost fully unionized in the intestine, which sounds ideal, but many weak bases dissolve well in the acidic stomach and then precipitate when they hit the near-neutral small intestine. Dasatinib and erlotinib are weak bases that are highly soluble in the acidic stomach and poorly soluble in the small intestine above pH 6.0, so they supersaturate and precipitate as they move from stomach to intestine [7]. Weak acids have the opposite problem: they dissolve poorly at low pH but dissolve well in the intestine, so their absorption is often limited by how fast the tablet disintegrates and how long the stomach holds it.

## What Modifies Absorption in Practice

### Food and Gastric pH

Food changes gastric pH, gastric emptying rate, bile flow and luminal volume. A physiologically based pharmacokinetic model that compared a fixed gastric pH of 5 against a multicompartment stomach capturing the changing pH profile after a meal found that the simpler model underpredicted posaconazole exposure in the fed state, while predictions for ibuprofen sodium (a weak acid) were comparable between models [8]. The takeaway is that weak bases are far more sensitive to the dynamic gastric pH profile after eating than weak acids are. For a weak acid, food mainly delays gastric emptying, which delays the time to peak concentration without necessarily changing total exposure.

### Gastric Emptying Rate

Gastric emptying is the rate-limiting step for absorption of most orally administered drugs, because the small intestine is where absorption actually happens. A study of raltegravir potassium, a poorly soluble acidic drug, found that dissolution from tablets was slow and was further delayed by pre-exposure to acid in the stomach. Plasma concentration profiles were successfully simulated only when gastric emptying time was treated as a key physiological variable [9]. In clinical terms, a drug that sits in the stomach for hours because of a full meal, ileus, or an opioid will have a delayed onset regardless of how soluble it is.

### Formulation

Formulation can override pH effects. A suspension of raltegravir potassium dissolved rapidly, while a tablet of the same salt dissolved slowly and was delayed by gastric acid exposure [9]. For weak bases, pH-independent controlled-release formulations of albendazole and dipyridamole maintained their elution and predicted absorption even when gastric pH was raised to achlorhydric levels, whereas physical mixtures and commercial tablets were extremely pH-dependent [10]. Salts and co-crystals of weak bases dissolve to a greater extent in hypochlorhydric and achlorhydric media than the free form's pH-solubility profile would predict, which is why biorelevant media are now used to model these interactions [11].

### Surfactants and Bile

Bile salts and dietary surfactants solubilize poorly soluble weak acids in micelles. A study of ketoprofen, a water-poorly soluble weak acid, showed that the micellar solubilization coefficient for the unionized form was 757 ± 165 L/mol, while for the ionized form it was only 9.88 ± 6.70 L/mol [3]. In plain terms, micelles carry mostly the unionized drug. This is one reason a fatty meal can increase absorption of a lipophilic weak acid even though it also slows gastric emptying.

### P-Glycoprotein Efflux

P-glycoprotein is an ATP-dependent efflux transporter in the apical membrane of enterocytes that pumps drug back into the intestinal lumen. It reduces net absorption of many weakly basic and neutral drugs. For weak acids, the effect is more variable because the anionic form is generally a poor P-gp substrate, but lipophilic weak acids that remain partly unionized in the enterocyte can still be effluxed. The clinical consequence is that P-gp inhibitors (ketoconazole, verapamil, cyclosporine) can raise plasma concentrations of co-administered P-gp substrates, and P-gp inducers (rifampin, St. John's wort) can lower them.

### Ion Trapping Beyond the Gut

Ion trapping is not confined to the gastrointestinal tract. It explains why weak acids are excreted faster in alkaline urine and weak bases faster in acid urine, a principle used in the treatment of poisoning and in drug monitoring [1]. It also explains the enormous volumes of distribution of lysosomotropic weak bases such as azithromycin and hydroxychloroquine, which accumulate in acidic lysosomes [2].

## How Absorption Is Measured and Modeled

Three approaches dominate.

**In vitro dissolution testing.** Standard USP apparatus I and II use a single vessel at fixed pH, which is a poor model for ionizable drugs because it cannot capture the pH shift from stomach to intestine [7]. Newer systems such as the mini-Gastrointestinal Simulator and the multicompartment gastrointestinal simulator reproduce gastric and intestinal compartments in sequence, allowing pH-dependent dissolution to be observed as it happens [7][12].

**Biorelevant media.** Media that mimic fasted and fed gastric and intestinal fluids, including hypochlorhydric and achlorhydric versions, are used to measure solubility and dissolution under conditions that match real patients on proton pump inhibitors or H2 blockers [11].

**Physiologically based pharmacokinetic modeling.** PBPK and PBBM models integrate dissolution, gastric emptying, intestinal transit, permeability and first-pass metabolism to predict plasma concentration profiles. These models can now capture gastric reacidification after a meal and predict food effects on weak bases [8]. They are used to support formulation design and bioequivalence decisions.

## Clinical Relevance, Limitations and Common Mistakes

The single most common student error is to conclude that a weak acid is absorbed mainly in the stomach because it is unionized there. It is not. The stomach's surface area is small and its residence time is short. The small intestine absorbs the majority of nearly every oral weak acid, because surface area and contact time dominate the ionization fraction [4].

The second common error is to assume that raising gastric pH with an antacid will reduce absorption of a weak acid. In practice, the effect is usually modest, because the weak acid was going to be absorbed in the intestine anyway. The drugs that suffer most from acid-reducing agents are weak bases, which depend on gastric acid for dissolution [12][13].

The third error is to treat pKa as a fixed clinical number rather than a physicochemical property. Two weak acids with the same pKa can have very different absorption profiles if one is highly lipophilic and the other is not, because membrane permeability depends on lipophilicity as well as ionization.

The fourth error is to ignore formulation. A salt of a weak acid can dissolve faster than the free acid, but it can also precipitate in the intestine if it supersaturates and does not revert to equilibrium solubility [5]. Formulation scientists exploit this deliberately, but it makes oral bioavailability less predictable than the pKa alone would suggest.

Finally, species differences in gastric pH, gastric emptying and intestinal surface area mean that a pharmacokinetic prediction from one species does not transfer directly to another. Pigs, dogs, cats and horses have different gastric anatomy and emptying patterns, and the same weak acid can behave differently in each. Individual cases require veterinary assessment.

## Quick Review

1. Only the unionized, lipid-soluble form of a weak acid crosses membranes by passive diffusion.
2. The Henderson-Hasselbalch relation gives the unionized fraction from pH and pKa.
3. A weak acid with pKa 3.5 is about 99% unionized at gastric pH 1.5 and about 0.1% unionized at intestinal pH 6.5.
4. The small intestine absorbs most of an oral weak acid dose because surface area and residence time outweigh the low unionized fraction.
5. Food, gastric emptying rate, formulation and P-glycoprotein efflux all modify absorption independently of pKa.
6. Ion trapping concentrates weak acids in alkaline compartments and weak bases in acidic compartments, including lysosomes.
7. Weak bases, not weak acids, are the drugs most vulnerable to acid-reducing agents and food effects.

## Frequently Asked Questions

### Why is aspirin absorbed in the intestine if it is unionized in the stomach?

Because the small intestine has a far larger surface area and a longer contact time, and because continuous absorption of the small unionized fraction pulls the dissociation equilibrium toward more unionized drug.

### Does food change absorption of weak acids?

Food mainly delays gastric emptying, which delays the time to peak plasma concentration. Total exposure to a weak acid is often less affected than for a weak base, which depends on gastric acid for dissolution.

### What is the difference between pKa and pH?

pH is the acidity of the surrounding fluid and changes with diet, disease and drugs. pKa is a fixed property of the drug molecule and does not change.

### Can antacids stop a weak acid drug from working?

Usually not to a clinically important degree, because weak acids are absorbed mainly in the small intestine. Weak bases are the drugs whose absorption falls when gastric acid is suppressed.

### What is ion trapping?

Ion trapping is the accumulation of a drug in a compartment where it becomes ionized and cannot diffuse back out, such as a weak acid in alkaline fluid or a weak base in an acidic lysosome.

### Does formulation matter as much as pKa?

Yes. Salts, co-crystals, suspensions and controlled-release granules can change dissolution rate and precipitation behavior enough to alter absorption independently of the drug's pKa.

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## Sources

1. [Influence of acid-base balance on efficacy and toxicity of drugs.](https://pubmed.ncbi.nlm.nih.gov/5854429/)
2. [Excessive lysosomal ion-trapping of hydroxychloroquine and azithromycin.](https://pubmed.ncbi.nlm.nih.gov/32389720/)
3. [Solubilization and dissolution of insoluble weak acid, ketoprofen: effects of pH combined with surfactant.](https://pubmed.ncbi.nlm.nih.gov/16982177/)
4. [Absorption of glibenclamide from different sites of the gastro-intestinal tract.](https://pubmed.ncbi.nlm.nih.gov/3935463/)
5. [Identification of key factors affecting the oral absorption of salts of lipophilic weak acids: a case example.](https://pubmed.ncbi.nlm.nih.gov/25252222/)
6. [Diffusion of weak acids across lipid bilayer membranes: effects of chemical reactions in the unstirred layers.](https://pubmed.ncbi.nlm.nih.gov/4752218/)
7. [In vitro dissolution methodology, mini-Gastrointestinal Simulator (mGIS), predicts better in vivo dissolution of a weak base drug, dasatinib.](https://pubmed.ncbi.nlm.nih.gov/25978875/)
8. [A multicompartment stomach physiologically-based pharmacokinetic model capturing gastric reacidification can improve food effect predictions of weak bases.](https://pubmed.ncbi.nlm.nih.gov/41490827/)
9. [Simulation of oral absorption from non-bioequivalent dosage forms of the salt of raltegravir, a poorly soluble acidic drug, using a physiologically based biopharmaceutical modeling (PBBM) approach.](https://pubmed.ncbi.nlm.nih.gov/33122010/)
10. [The use of an in vitro dissolution and absorption system to evaluate oral absorption of two weak bases in pH-independent controlled-release formulations.](https://pubmed.ncbi.nlm.nih.gov/15961297/)
11. [Physiologically Based Absorption Modeling of Salts of Weak Bases Based on Data in Hypochlorhydric and Achlorhydric Biorelevant Media.](https://pubmed.ncbi.nlm.nih.gov/29872977/)
12. [Use of Gastrointestinal Simulator, Mass Transport Analysis, and Absorption Simulation to Investigate the Impact of pH Modifiers in Mitigating Weakly Basic Drugs' Performance Issues Related to Gastric pH: Palbociclib Case Study.](https://pubmed.ncbi.nlm.nih.gov/36367432/)
13. [Improving Dissolution Behavior and Oral Absorption of Drugs with pH-Dependent Solubility Using pH Modifiers: A Physiologically Realistic Mass Transport Analysis.](https://pubmed.ncbi.nlm.nih.gov/34428047/)