Why Do Some Medications Come As Pills, And Others As Injections?

assortment of medicines
Katie Mui
Katie Mui is on the Research Team at GoodRx.
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Why do some medications come in tablets and others in capsules? Why are there ointments and creams? And why are some drugs delivered by injection or through an intravenous (IV) drip?   

Like a lot of things in medicine, the answer can get complicated, but it boils down to this: where a drug needs to be, how quickly it needs to get there, and how long it needs to hang around. After a major surgery, your doctor could prescribe a powerful painkiller by IV drip, which gets the drug circulating right away, and to pain receptors throughout your body. But if you have a chronic condition, IV isn’t convenient – you probably don’t have time to sit for an hour after breakfast and before bedtime while medication drips slowly into your veins. So an oral formulation, like a tablet or a capsule, is better.

To get a sense of which formulations work for which purposes, it helps to think of the form — a pill, an ointment, or an injectable — as a vehicle, and the drug itself as a passenger along for the ride. Here’s a primer.

The passenger: The drug

The most important part of a medication is its active ingredient, such as ibuprofen for pain relief or rosuvastatin for cholesterol control. It is the drug (or chemical) that creates a therapeutic response in your body. How the drug gets where it’s going safely, and at the right speed, is the job of the vehicle, made of non-therapeutic inactive ingredients (also called excipients) that facilitate transport and uptake.

The vehicle: The dosage form

In a tablet, the drug is combined with substances to give the tablet its shape, texture, and color. These inactive ingredients keep the drug chemically stable so it can be shipped and stored, and they help dictate the way the pill is digested in your stomach and intestines, freeing the active ingredient. Tablets are durable, which is why so many medications come in tablet form. They can withstand the physical pressures of a pill bottle or your pocket, protecting the drug. They’re kind of like the transport truck of dosage forms.

An injectable medication, on the other hand, conveys the drug in a liquid form along with inactive ingredients. After injection, the fluid flows readily into the bloodstream (or body tissues) and the drug is easily freed. It’s more like a motorcycle: there’s not much protection for the passenger, but the ride tends to be faster and purer.

The road: Our bodies

Each drug needs to get from where you take it (the site of administration) to its ultimate intended destination (the site of action). The site of action might be as localized as a patch of skin or a particular organ, or as dispersed as chemical receptors found throughout your body. For example, morphine, a strong painkiller, acts on receptors found in the brain, spinal cord, limbic system, and even in your digestive tract.

Many medications need to get into the general circulation of fluids in your body — the bloodstream, in particular — in order to flow to their sites of action. Ideally, 100% of a drug gets to its destination. In practice, most medications need to pass through multiple barriers in your body (biologic membranes) and undergo a complex series of biochemical reactions along the way, some of which “use up” a portion of the active ingredient.

When a medication is given intravenously, it enters the circulation directly and is considered 100% available to your body. When taken by mouth, it has to go through part of your digestive system first. Only some of the drug makes it to the site of action; the rest is effectively processed as waste. This concept is called bioavailability: the portion of the drug that actually makes it into your circulation.

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Choosing the right ride: Different forms for drugs

When scientists create medications, they choose a dosage form that marries biochemistry with convenience. The drug needs to do its job effectively and safely, but the medication has got to be practical, too. There are dozens of common routes of administration and dozens of dosage forms: patches that deliver drugs through the skin, solutions that are injected not just into veins but into muscles, beneath the skin or directly into organs, and liquids that are meant to be dropped into eyes and ears.

Oral forms

The most common oral dosage forms are tablets, capsules, and liquids.

Nasal and oral inhalation

Breathing a drug in through your nose or mouth is a quick way to get a drug into your system: your airway and lungs have a lot of surface area to absorb the drug. Some drugs go to work right in the airway, so inhaling them gets them where they need to go, such as drugs that treat bronchial problems or asthma.

Drugs packaged for inhalation may come in liquid, solid, or gaseous form, assisted by all manner of useful excipients. Nasal anti-inflammatory drugs like fluticasone are distributed in spritzers that mist droplets of the drug into your nose for inhalation. Inhalers for treating asthma commonly employ two forms: the drug aerosolized in a chamber along with a propellant that forces it to spray out of the inhaler, or as a fine dust (dry powder). Anesthetic drugs used to “put you to sleep” before surgery are frequently given in gaseous form — a few breaths and you’re out cold. Laughing gas, or nitrous oxide, is another drug famously given as a gas.

Topical

Topical medications — commonly liquids, creams, or ointments — are applied directly to a surface, like your skin, right at the spot they’re needed. And because they don’t go through the digestive system, they tend to have fewer systemic effects or side effects.

Many ophthalmic drugs are also topical — for obvious reasons, liquid is a suitable form for eye drops. But why can you get the same medication, like clobetasol for treatment of psoriasis, in all three forms? Well, you wouldn’t want to rub a cream or ointment into your scalp if you’ve got a full head of hair. And while water-based creams readily absorb, getting the drug into the skin quickly, your particular condition might need a slow-and-steady release of medication; an oil-based ointment doesn’t absorb so easily and will “hang around” longer, letting the drug diffuse more slowly into the skin.

Patches

Transdermal patches are another form of topical medication, commonly used for nitroglycerin to treat angina, nicotine patches to help people quit smoking, and for some pain and birth control drugs. They may deliver a drug close to where it’s needed, but more commonly are used to deliver a steady dose of medication over a lengthy period of time — one that doesn’t go through your digestive system. Because these drugs bypass metabolism in the liver, this form is suitable for drugs like nitroglycerin, ensuring high bioavailability.

Injection

Many drugs are administered by injection. They’re distributed in a liquid solution or suspension and injected into a vein, organ, or tissue — wherever the drug is needed, or the best place to get it circulating. 

Other drugs are injected between layers of skin (intradermal), into muscle tissue (intramuscular), or, in hospitals, even directly into organs like the heart (intracardiac) or brain (intracerebral). An ophthalmologist might even inject a medication directly into the cornea of the eye, in an intracorneal injection. These injections get a dose of drug straight to the site of action.

What does this all mean?

As you can see, medications come in many forms and there are just as many reasons for such. There is a whole discipline — pharmacokinetics — concerned with the biochemical journeys taken by drugs.

But for the average patient, the only time we need to think about different medication forms is if the treatment we were prescribed is not working well enough or if it’s too expensive. Not every medication comes in multiple forms, but you can ask your doctor (or check GoodRx) if they have another recommendation for you.

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