How Medicines Actually Work Inside Your Body

Author: Dr. Priya Mehta, Health and Wellness Writer

A few years ago, I sat with a friend who’d just been diagnosed with high blood pressure. Her doctor had prescribed her a daily tablet, and she looked at it sitting in her palm with genuine confusion. “How does this tiny thing even know where to go?” she asked. “How does it know my blood pressure is the problem and not my knee or something?”

It’s such a good question. And honestly, not enough people ask it. We’re pretty good at taking medicines when we’re told to, but most of us have very little idea what happens after they disappear down the throat. The whole process feels like a black box.

It doesn’t have to be. The way medicines work is genuinely fascinating once you understand the basics, and knowing how they function can actually make you better at taking them. You start to understand why timing matters, why some tablets need to be taken with food, why you can’t just stop certain medications abruptly, and why the same dose affects different people differently.

So let’s open that black box.

The Journey Begins: Getting Into the Body

Before a medicine can do anything useful, it has to get into your bloodstream. The route it takes determines a lot about how quickly it works and how much of it actually reaches the target.

The most common route is swallowing a tablet or capsule. From there, the medicine travels to your stomach, then your small intestine, where most of the absorption happens. The lining of the small intestine is remarkably efficient at pulling molecules across into the bloodstream, and that’s where most oral medicines make their entry into circulation.

But getting to the small intestine doesn’t automatically mean getting absorbed. Some medicines are broken down by stomach acid before they even get there, which is why certain tablets are coated with a protective layer designed to survive the stomach and only dissolve in the more neutral environment of the small intestine. These are called enteric-coated tablets, and crushing or chewing them defeats the whole purpose.

Some medicines bypass the digestive system entirely. Injections deliver the drug directly into the bloodstream, or into muscle tissue where it’s gradually absorbed. Patches release medicine slowly through the skin. Inhalers deposit medicine directly into the airways. Each method has a reason, usually related to how the medicine behaves chemically, how quickly it needs to work, or whether it would survive the digestive process at all.

The First-Pass Effect: The Liver’s Role

Here’s something most people don’t know about oral medicines, and it has a big impact on how medicines are dosed.

After absorption from the small intestine, blood doesn’t flow directly to the rest of the body. It goes to the liver first, through a system called the portal circulation. The liver is the body’s main chemical processing plant, and it treats many medicines as things to be broken down and neutralised. This process is called the first-pass effect, and for some medicines it dramatically reduces the amount of active drug that actually makes it into general circulation.

This is why the dose of an oral medicine might be considerably higher than a dose delivered directly into the bloodstream by injection. The oral dose has to survive the liver’s first attempt to deactivate it. For some medicines, the first-pass effect is so significant that oral delivery simply doesn’t work at all. The liver gets to it too efficiently and nothing useful reaches the target. These medicines have to be delivered another way.

The liver processes medicines using specialised enzymes, particularly a family called cytochrome P450 enzymes. This is also why drug interactions happen. When two medicines use the same liver enzymes, they compete with each other. One might slow down the breakdown of the other, causing it to accumulate to higher-than-expected levels. Or one might speed up the enzyme activity, causing the other to be cleared too quickly and become ineffective. Grapefruit juice famously interferes with some of these enzymes, which is why certain medications specifically warn against drinking it.

How Medicines Find Their Targets

Once in the bloodstream, a medicine gets distributed throughout the body. Blood goes everywhere, so in theory the medicine does too. But medicines don’t act everywhere equally. They’re designed to work at specific locations, and the way they find those locations is one of the most elegant aspects of pharmacology.

Most medicines work by binding to specific molecules on or inside cells called receptors. Receptors are essentially the body’s communication infrastructure. They’re proteins designed to receive signals and trigger a response. Hormones, neurotransmitters, and other chemical messengers work by binding to receptors and activating them.

Medicines exploit this system in a few key ways.

Agonists are medicines that bind to a receptor and activate it, mimicking the effect of the body’s own signalling molecules. Salbutamol, used in asthma inhalers, binds to receptors in the airway muscles and tells them to relax, widening the airways. It’s essentially impersonating one of the body’s own bronchodilating signals.

Antagonists are medicines that bind to a receptor and block it, without activating it. They sit in the receptor like a key in a lock that turns nothing, preventing the body’s natural signals from getting through. Beta blockers, used for heart conditions and high blood pressure, work by blocking the receptors that normally respond to adrenaline. By sitting in those receptors, they prevent adrenaline from speeding up the heart and raising blood pressure.

Enzyme inhibitors work differently. Instead of targeting receptors, they interfere with enzymes, the proteins that drive chemical reactions in the body. Statins, used to lower cholesterol, inhibit an enzyme involved in the body’s own cholesterol production pathway. By blocking that step, they reduce how much cholesterol the liver produces. ACE inhibitors, used for blood pressure and heart failure, block an enzyme involved in producing a hormone that narrows blood vessels.

The reason a medicine affects your blood pressure and not your knee, to return to my friend’s question, is that its target receptors or enzymes are predominantly found in the tissues relevant to blood pressure regulation. The medicine doesn’t navigate purposefully. It floods the whole system, but it only sticks where its specific binding site exists.

What the Body Does to the Medicine: Metabolism

The body doesn’t just passively carry medicines around. It’s actively trying to process and eliminate them, which is exactly what you’d want for any foreign chemical that enters the system.

Metabolism is the process of chemically transforming a medicine into different compounds, generally ones that are easier to excrete. The liver does most of this work, though the kidneys, lungs, gut, and other tissues also contribute.

Metabolism can do one of several things. It can inactivate a medicine, converting an active drug into something that no longer has a biological effect. This is the normal endpoint for most medicines. It can also produce active metabolites, compounds that are themselves pharmacologically active. Some medicines are actually designed as prodrugs, meaning they’re pharmacologically inactive until the body metabolises them into their active form. Codeine, for example, is converted by the liver into morphine, which is the compound that actually provides pain relief.

The rate at which medicines are metabolised varies significantly between people, and this explains a lot of the individual variation in how medicines work. Genetic differences in metabolising enzymes mean that some people process certain medicines very quickly, potentially too quickly for them to be effective at standard doses. Others process them slowly, leading to higher-than-expected blood levels and potentially stronger effects or more side effects. This field, called pharmacogenomics, is increasingly being used to personalise medicine dosing.

Age matters too. Newborns have immature metabolic systems and clear medicines slowly. Elderly people often have reduced liver function and kidney function, meaning medicines stay in the body longer. This is why dosing recommendations in older adults are often more conservative.

Excretion: Leaving the Body

After metabolism, the breakdown products of a medicine need to leave the body. The kidneys are the primary route for most medicines and their metabolites, filtering them out of the blood and excreting them in urine. This is why kidney disease can significantly affect how medicines behave in the body. If the kidneys aren’t working efficiently, drug levels can build up to unsafe concentrations.

Some medicines are excreted through bile into the gut and leave via the faeces. A small number are excreted through the lungs, which is why you can smell certain things like garlic on someone’s breath after they eat it in large quantities. The same principle applies to some medicines.

The concept of half-life is important here. The half-life of a medicine is the time it takes for the concentration in the body to fall by half. A medicine with a short half-life leaves the body quickly and needs to be taken more frequently to maintain effective levels. A medicine with a long half-life persists in the body for days or even weeks, which means it can be taken less frequently but also means that if something goes wrong, it takes longer to clear the system.

This is why some medicines need to be taken multiple times a day while others are once-weekly or even once-monthly. The dosing frequency is a function of the half-life, not just the effect duration.

Why Timing and Food Matter

Now that you understand the journey, the practical advice about how to take medicines starts to make a lot more sense.

Taking some medicines with food slows their absorption, which can be useful if you want a more gradual, sustained effect. For others, food interferes with absorption because the active ingredient binds to certain components in food rather than being absorbed. Some medicines are irritating to the stomach lining and food provides a protective buffer.

Taking medicines at consistent times each day helps maintain stable blood levels, which matters more for some medicines than others. For medicines that need to maintain a therapeutic concentration in the blood to be effective, irregular timing creates peaks and troughs that can reduce effectiveness or increase side effects.

Some medicines should never be crushed or chewed, not because of taste, but because the tablet design is integral to how the medicine is released and absorbed. Modified-release formulations are designed to dissolve slowly over hours, providing a steady level of medicine. Crushing them releases everything at once, creating a spike that the body wasn’t designed to handle.

Side Effects: Why They Happen

Understanding how medicines work also explains why side effects happen. Receptors and enzymes are rarely completely specific to one tissue or one function. A receptor that’s primarily associated with blood pressure regulation might also be present in other tissues. When a medicine blocks that receptor throughout the body, effects happen everywhere it exists, not just at the intended site.

Beta blockers lower heart rate and blood pressure by blocking adrenaline receptors in the heart and blood vessels, but adrenaline receptors also exist in the lungs. In people with asthma, blocking those receptors can trigger bronchoconstriction, which is why beta blockers are generally avoided in asthma patients. Same mechanism, same receptor, different tissue, unintended effect.

Some side effects come from the medicine’s metabolism. If a metabolite has its own biological activity, it can produce effects that the parent compound doesn’t.

Some side effects are dose-dependent. The therapeutic effect and the toxic effect are both produced by the same mechanism, just at different concentrations. The goal in dosing is to find the amount that produces the intended effect without reaching the concentration that causes harm. That balance point varies between individuals.

Drug Interactions: When Medicines Affect Each Other

When you take two or more medicines at the same time, they can influence each other’s behaviour in ways that matter clinically.

Pharmacokinetic interactions affect how a medicine moves through the body. One medicine speeds up or slows down the metabolism of another, changing the effective dose. This is common with liver enzyme interactions and is one of the main reasons healthcare providers need a complete list of everything you’re taking, including over-the-counter medicines and supplements.

Pharmacodynamic interactions affect what medicines do rather than how they’re processed. Two medicines with similar effects can amplify each other dangerously. Two medicines with opposing effects can cancel each other out. Blood-thinning medicines and anti-inflammatory drugs both affect bleeding risk through different mechanisms, and combining them requires careful consideration.

This is why the instruction to tell your doctor and pharmacist about all medicines and supplements you take isn’t just a formality. It’s a practical safety measure rooted in real pharmacology.

Why the Same Medicine Affects People Differently

This is probably the most common frustration people have with medicines. The same tablet that works perfectly for one person does nothing for another, or causes side effects that a different person never experiences.

Genetics explains a significant part of this. Variations in the genes that code for metabolising enzymes mean that populations metabolise medicines at different rates. Variations in receptor structure can make some people more or less sensitive to a medicine’s effects.

Body weight and composition affect distribution. A medicine that distributes into fat tissue will have a very different effective concentration in someone with more fat tissue compared to someone with less, even at the same dose.

Age, kidney function, liver function, other medicines being taken, diet, and even gut bacteria all influence how medicines are processed. Medicine is dealing with the enormous biological variability of human beings, and the doses and protocols that work for most people are based on population averages. Individual variation is real and expected.

This is why working with a doctor rather than self-adjusting doses based on how you feel is important. What feels like a medicine not working might be a dosing issue, a timing issue, an interaction, or a different underlying problem that the medicine wasn’t designed for. The pharmacology is complicated, and interpreting it well requires professional training.

A Word on Where to Find Reliable Information

For Australians wanting to understand specific medicines better, NPS MedicineWise at nps.org.au is one of the best resources available. It’s funded by the Australian government and provides accurate, independent, consumer-friendly information about how specific medicines work, what to expect, and when to seek advice. The healthdirect service at healthdirect.gov.au also provides clear medicine information backed by clinical review.

Understanding how medicines work doesn’t make you your own pharmacist or doctor. But it does make you a more informed patient, someone who takes medicines correctly, notices when something seems off, and has better conversations with the people responsible for their care. That combination of professional expertise and informed patient engagement is genuinely one of the most effective things in healthcare.

That tiny tablet my friend was holding knows exactly where to go. It just needed someone to explain the journey.