Bioavailability: Why the Same Drug Works Differently Depending on How It’s Delivered

Introduction

There are situations where a person may take a tablet for a health condition, while another may receive an injection for the same issue. Although the drug, and sometimes even the dose, may be identical, the outcomes can differ.

This difference often comes down to how the drug is delivered. In more complex or severe conditions, injections are preferred because they allow the drug to act more rapidly and reliably compared to oral forms.

This is where the concept of bioavailability becomes important. It highlights that the effectiveness of a treatment depends not only on the drug itself, but also on how it enters the body and reaches its site of action.

What Is Bioavailability?

Bioavailability refers to the extent and rate at which an active ingredient from a medicine is absorbed and becomes available in the body. [1] It describes the proportion of a drug that reaches its intended site of action or enters systemic circulation in an unchanged form. [2] In most cases, it is defined as the fraction of the administered dose that successfully reaches the bloodstream and is available to produce a therapeutic effect.

Bioavailability varies depending on the route of administration, including oral, topical, parenteral, and rectal routes. It reflects both how much of the drug enters circulation and how quickly it does so. For example, orally administered drugs often have lower bioavailability than intravenous drugs due to incomplete absorption and first-pass metabolism in the liver. [3]

Routes of Drug Delivery

Bioavailability is a key component of pharmacokinetics, which describes how drugs move through the body, including their absorption, distribution, metabolism, and elimination (ADME). [2][4] Drugs given intravenously enter systemic circulation directly, undergoing only distribution and elimination. [4] The choice of administration route depends on factors such as drug properties, pharmacokinetics, and clinical needs. [5]

Oral Administration

Most orally administered drugs are absorbed in the small intestine, and their bioavailability depends on how well they cross the intestinal lining. However, drug levels may be significantly reduced before reaching systemic circulation due to first-pass metabolism in the liver. [6] Although convenient, oral administration can be limited by poor solubility, low stability in the gastrointestinal tract, reduced permeability, and systemic side effects. [7]

Intravenous Administration

Intravenous (IV) administration delivers drugs directly into systemic circulation, bypassing first-pass metabolism and resulting in 100% bioavailability. [5][9] It is used when rapid effects, precise drug levels, or reliable absorption are required, particularly in patients unable to take oral medications. [5] Drugs may be given as a bolus or continuous infusion, depending on the desired duration of action. [8]

Intramuscular / Subcutaneous Administration

Intramuscular (IM) injections deliver drugs into well-vascularised muscle, allowing relatively rapid absorption into systemic circulation while bypassing first-pass metabolism. [10][11] However, site selection is important due to potential risks such as nerve or vascular injury. [5]

Subcutaneous (SC) injections are administered beneath the skin and are absorbed more slowly due to lower blood supply, resulting in sustained drug release. [5] Both IM and SC routes involve absorption from the injection site, which can delay onset of action. [12]

Transdermal Administration

The transdermal route delivers drugs through the skin using patches, gels, or creams. [5] It bypasses the gastrointestinal tract and first-pass metabolism, making it useful for drugs with poor oral bioavailability. [13] This method also allows for sustained drug release and improved patient compliance. [14]

Inhalation

Inhaled drugs are rapidly absorbed through the large surface area of the respiratory tract and enter systemic circulation directly, avoiding first-pass metabolism. [5] This route is commonly used for respiratory conditions such as asthma and chronic obstructive pulmonary disease. [15] Inhalation methods include nasal, pulmonary, steam, and anesthetic delivery. [16]

The choice of drug administration route depends on multiple factors, including drug properties, desired effect, patient characteristics, and pharmacokinetics. [16]

What Happens Inside the Body

Drug goes through a variety of processes, the most importance of which are as follows:

Absorption

Drug absorption is the process by which an unmetabolised drug moves from its site of administration into the bloodstream. [19] The most common mechanism is passive diffusion, where drug molecules move from areas of high concentration to low concentration until equilibrium is reached. [17] Drugs can also cross membranes via carrier-mediated transport systems, including facilitated and active transport. [17]

Several physiological factors influence absorption, including age, gastric emptying time, intestinal transit, blood flow, disease state, and gastrointestinal contents. [17] The small intestine is the primary site of absorption due to its large surface area. However, orally administered drugs may be metabolised in the gut wall or liver before reaching circulation, reducing the amount of active drug available, a process known as first-pass metabolism. [17]

Food intake, particularly fatty meals, can slow gastric emptying and delay drug absorption. [18] While intravenous drugs bypass absorption and achieve 100% bioavailability, oral drugs are more variable but remain the most commonly used route due to convenience. [17] Drugs may cross membranes through passive diffusion, facilitated diffusion, active transport, or pinocytosis. [18]

First-Pass Metabolism

First-pass metabolism refers to the breakdown of a drug before it reaches systemic circulation, primarily occurring in the liver and, to a lesser extent, in the gut wall and other tissues. [19][20] After oral administration, drugs travel via the portal vein to the liver, where enzymes may metabolise and inactivate a portion of the drug before it enters circulation. [21]

This effect reduces the concentration of active drug reaching its target and often requires higher oral doses compared to intravenous administration. [20] First-pass metabolism can also occur in other tissues such as the lungs and gastrointestinal tract. [20] To bypass this effect, alternative routes such as sublingual, nasal, rectal, or transdermal delivery may be used. [19] Overall, first-pass metabolism significantly decreases the amount of drug available at the site of action. [21]

Barriers and Distribution

Bioavailability is influenced by factors that limit a drug’s ability to reach systemic circulation, including physiological barriers, transport systems, and early metabolism. [22] For oral drugs, barriers include poor solubility, low membrane permeability, instability in the gastrointestinal tract, and interactions with pH, enzymes, and efflux transporters. [23]

Once in circulation, drugs are distributed throughout the body’s tissues. Distribution depends on factors such as blood flow, tissue type, protein binding, pH, and the drug’s chemical properties. Many drugs bind to plasma proteins like albumin and globulins; however, only the unbound (free) drug can leave the bloodstream and interact with target tissues. Differences in distribution across tissues such as fat, muscle, and brain contribute to variations in drug effectiveness and toxicity. [24]

Why This Matters

Understanding bioavailability is essential for determining the correct drug dosage. Drugs with low bioavailability may require higher doses to achieve a therapeutic effect, while higher bioavailability can increase the risk of toxicity. The fraction of a drug that reaches systemic circulation directly influences how effective it will be in the body. [25]

In clinical practice, bioavailability guides the choice of route of administration to maximise drug effectiveness. Variations in bioavailability, due to genetic differences, disease, or drug interactions, can lead to reduced efficacy or increased toxicity. [23]

Bioavailability is commonly expressed as:

F = Amount of active drug in systemic circulation / Amount of drug administered [23]

Understanding this concept helps explain why the same drug can produce different outcomes depending on how it is delivered and how the body processes it.

Conclusion

While the choice of a drug plays a key role in treatment decisions, bioavailability determines how effectively that drug can act within the body. It influences the onset, intensity, and duration of a drug’s effect, ultimately guiding how therapies are designed and administered.

References

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