Subcutaneous vs intramuscular injection

How the two main injection routes change absorption rate, depot behaviour and the shape of the modelled curve — and why a route change is not the same as a dose change.

Subcutaneous (SC) and intramuscular (IM) injection deliver the same molecule into very different tissue environments. The route changes how fast the drug is absorbed, how prolonged the depot effect is, and — for some compounds — how high the peak serum concentration ends up. It does not, in most cases, change the total area under the curve at the same dose. This page covers what changes between routes, what doesn't, and how to think about a route switch through the lens of the DoseCurve chart.

This is educational background reading. It is not a recommendation about which route to use, and it does not replace clinical guidance from someone who knows your history.

The two tissue environments

Subcutaneous tissue is the layer of fat and loose connective tissue immediately beneath the skin. It is poorly vascularised compared to muscle, which means absorption from a SC depot is generally slower and more sustained.

Intramuscular tissue is the bulk of skeletal muscle — well vascularised, well perfused, and capable of taking larger injection volumes without discomfort. Drugs injected IM typically reach the systemic circulation faster than the same drug injected SC.

For an oil-based depot ester, both routes form a depot. The depot in subcutaneous fat is generally smaller in cross-section and slower to release; the depot in muscle is more vascularised and releases faster. For a water-based formulation (insulin, GLP-1s, most peptides) the difference is similar — SC absorption is slower and more sustained than IM.

What the literature shows for testosterone

The clearest body of evidence comparing routes is in testosterone replacement. Multiple controlled studies — including Spratt et al. (2017), Frasca et al. (2019) and the registration trial for the SC-administered Xyosted formulation — have found that subcutaneous testosterone enanthate or cypionate produces:

Similar average serum testosterone levels at the same weekly dose as intramuscular.
Lower peak serum levels and higher trough serum levels, i.e. a flatter pharmacokinetic profile.
Comparable or slightly improved patient-reported tolerability.
A different injection-site adverse event profile (more local reactions, fewer post-injection coughs or pain spikes).

In other words: same total exposure, smoother curve. That is exactly what DoseCurve would predict if you increased the effective half-life slightly while keeping the same weekly milligrams.

What this looks like on the chart

Within the limits of a single-compartment exponential model, a slower release rate is equivalent to a longer effective half-life. Try this in DoseCurve: set up two profiles at the same weekly dose, with the second profile having a moderately longer half-life. The flatter line is the SC curve in caricature. The same average, lower peaks, higher troughs.

The model cannot capture the absorption phase itself — that early ramp-up after an injection that takes hours for IM and longer for SC. A two-compartment PK model with an absorption rate constant would; DoseCurve does not. For most users the simplification is acceptable because the question of interest is "what does the steady-state shape look like over weeks", not "what is my serum level four hours after injection".

Where the picture is different

Not every compound behaves the same way:

Insulin. The route choice for insulin is essentially fixed as subcutaneous (rapid- and long-acting analogues are formulated for SC). IM injection of insulin is faster and produces more variable absorption — clinically undesirable.
GLP-1 analogues (semaglutide, liraglutide, tirzepatide). All approved for SC administration only. IM is not the labelled route and would change the absorption kinetics in ways the registration data does not characterise.
Vaccines. Most modern vaccines are IM because the muscle tissue's antigen-presenting cell density produces a better immune response than SC.
Aqueous-suspension drugs. Some long-acting suspensions (e.g. depot antipsychotics, certain depot contraceptives) are specifically formulated for one route. Cross-route use changes the kinetics enough that it is not equivalent.
Oil-vehicle compounds with high injection volumes. A 3 mL oil bolus is comfortably IM in the gluteus medius. SC, the same volume often produces a visible bulge and prolonged local discomfort. Some users split into multiple SC injections; some switch to IM for high-volume doses; some adjust frequency. None of this is a recommendation — it is the practical trade-off space your clinician can speak to.

What the model can and cannot show

The DoseCurve chart can show:

The relative shape consequence of an effectively slower or faster release rate.
The peak-to-trough flattening that comes from a smoother depot.
The steady-state average and how long it takes to get there.

The chart cannot show:

The actual peak serum concentration on the day of injection (it does not model the absorption phase).
Local injection-site adverse effects.
Differences in patient-reported tolerability.
Vehicle-specific behaviour (oil vs aqueous, brand-specific excipients).

For any of those you need either the published clinical pharmacology data for your specific formulation or bloodwork.

Practical questions to take to a clinician

If you are weighing a route discussion with a prescriber, things they will usually want to consider:

The labelled route for your specific formulation. Some products are only studied at one route.
Local adverse-effect history. Persistent IM pain or post-injection coughs are not normal and may shift the discussion toward SC.
Total injection volume. Large oil-vehicle volumes are usually better tolerated IM.
Dose interval. Smoother SC profiles can sometimes support longer intervals at the same total weekly dose.
Patient preference and dexterity. SC injection is easier to self-administer in most non-clinical settings.

None of those decisions belong in a pharmacokinetic visualiser. Use the chart for shape; take the route decision to someone who knows you.