The pharmacokinetics of semaglutide: how it moves through your body

Step 1 — absorption: what happens in the hours after your injection

After a subcutaneous injection, semaglutide is absorbed from the tissue slowly and steadily. Bioavailability sits at around 89%, meaning the vast majority of each dose reaches the bloodstream intact. The injection site matters to a degree: the upper arm tends to produce a slightly faster rise to peak concentration than the abdomen or thigh, though all three are licensed sites and the clinical difference is modest. Peak concentration (Tmax) typically arrives one to three days post-dose.

This gradual absorption curve is intentional. A fast spike would amplify nausea; the slow rise is part of why the titration schedule (starting at 0.25 mg and stepping up every four weeks) reduces early gastrointestinal side effects. The semaglutide overview on this site covers what those early weeks tend to feel like in practice. At the tissue level, semaglutide binds extensively to albumin in the bloodstream, which slows its elimination and underpins the long half-life described in the next step.

Storage before injection matters too: if you are using Raw Pharma semaglutide, which is one of the branded formulations available through this pharmacy, the pen should be kept refrigerated between 2 and 8°C; always follow the instructions in the Patient Information Leaflet for the exact window at room temperature before the dose is given.

Step 2, distribution and half-life: why once-weekly dosing holds steady

Semaglutide's plasma half-life of approximately seven days is the single fact that makes a once-weekly schedule clinically workable. After each injection the drug level rises to a peak, then declines gradually across the rest of the week, reaching a trough just before the next dose is due. With repeated weekly injections, the body reaches a pharmacokinetic steady state after four to five weeks, meaning blood levels stop climbing from week to week and instead oscillate within a narrower range.

That steady state matters for two reasons. First, tolerability: many people find nausea most noticeable at the start of treatment, partly because the drug level is still climbing before reaching equilibrium. Second, missed doses: because the half-life is long, a single missed injection does not immediately clear semaglutide from the system. The Wegovy treatment guide explains what the prescriber's guidance says about missed doses, but the short version is that the timeline is more forgiving than with a daily medicine.

The seven-day half-life is also why a wash-out period is recommended before pregnancy, the drug lingers well beyond the last injection date, and current guidance advises stopping treatment and using reliable contraception in the interim. Speak to your prescriber about the specific period applicable to your circumstances.

Step 3, metabolism and elimination: how the body clears semaglutide

This is where semaglutide differs most strikingly from most familiar medicines. It is not broken down by the cytochrome P450 enzymes that process the majority of oral drugs and that underlie most food-and-medicine interactions. Instead, semaglutide is metabolised by proteolysis (sequential cleavage of the peptide chain by enzymes found throughout the body) and the resulting fragments are then excreted via both the urine and faeces.

Because no active metabolites are produced, you do not accumulate a different compound as treatment continues. The drug that leaves the body is simply a smaller fragment of the same peptide, pharmacologically inert. The pharmacology of semaglutide page covers how the GLP-1 receptor mechanism works upstream of this elimination stage.

The practical implication for drug interactions is important: most interactions with semaglutide relate to its slowing of gastric emptying rather than enzyme competition in the liver. Medicines that need to be absorbed quickly, or that have narrow therapeutic windows sensitive to timing, should be flagged to your prescriber. Oral contraceptives are one example the NHS highlights specifically for this reason. If you take other regular medicines, a prescriber can assess any implications as part of the standard consultation process.

For a closer look at how the subcutaneous route specifically affects absorption variables, the page on subcutaneous semaglutide pharmacokinetics goes into the injection-site data in more detail. You can also read the detailed prescribing information in the BNF entry for semaglutide, which is the reference UK prescribers use.

What these pharmacokinetics mean for starting or switching treatment

Knowing how semaglutide moves through the body also shapes the titration schedule. The 0.25 mg starting dose for Wegovy is not intended to produce meaningful weight loss by itself; its job is to let the body adapt to the drug's GI effects while the pharmacokinetics reach steady state. Dose steps happen every four weeks roughly because that is enough time for the body to re-equilibrate at each new level before the prescriber assesses whether to increase further.

People switching from one semaglutide formulation to another (for instance, from an injectable to the newer oral tablet) need clinical guidance on timing, partly because the pharmacokinetic profiles differ substantially. The oral form (Wegovy tablets, approved by the MHRA on 11 June 2026 as the first oral GLP-1 medicine licensed in the UK for weight management) has a much lower oral bioavailability than the injection, requiring a higher milligram dose and strict fasting rules to achieve adequate absorption. That is a different pharmacokinetic story covered on the Wegovy pharmacology page.

People sometimes ask about the timing of their injection around holidays or busy weeks. A dose can be taken a day or two earlier or later than the usual day without clinical concern, thanks to the long half-life, though no adjustment should be made without checking the Patient Information Leaflet first. It is one of the few practical perks of a medicine designed to hold steady across a full week. The NHS medicines page for semaglutide covers missed-dose guidance in plain language.

If questions about your own pharmacokinetics, interactions or titration timing have come up during treatment, a prescriber can work through them properly. Speak to our prescribers through a free consultation, and a GPhC-registered Independent Prescriber will review your case the same day. For a broader look at the evidence behind the medicine, the weight-loss treatments overview is a useful starting point.