Subcutaneous (subq) injection

A subcutaneous injection goes into the soft layer of fat between your skin and your muscle — not into a vein, and not deep into muscle. That layer has a modest blood supply, so the drug seeps into circulation slowly over hours or days instead of arriving all at once. That slow release is exactly what a once-weekly GLP-1 needs, and it is why the needles are short and thin enough that most people describe the shot as a pinch rather than pain.

The approved labels are specific about where. Wegovy, Zepbound and Saxenda all direct the injection into the abdomen, the thigh, or the upper arm, and all three tell you to rotate sites with each dose. The labels also confirm something patients worry about needlessly: exposure to the drug was similar whether semaglutide or tirzepatide was injected in the abdomen, the thigh or the arm. Picking the belly over the thigh does not make the medicine stronger.

The numbers behind that are reassuring. Roughly 89% of a subcutaneous semaglutide dose reaches the bloodstream, and peak blood levels arrive one to three days after the shot. For tirzepatide it is about 80%, peaking somewhere between 8 and 72 hours. So a Thursday injection is still working on Sunday — the medication is not something that hits and fades within hours.

The most common mistake is injecting into the same square inch week after week. Repeated trauma to one spot can thicken the tissue and make absorption unpredictable, which is the problem described under lipohypertrophy. The Saxenda label adds a second reason to rotate: reducing the risk of cutaneous amyloidosis, a lump of protein deposit at an overused site. Our injection-site rotation calendar and the site-comparison evidence review lay out a workable pattern.

Worth asking your clinician or pharmacist: whether to pinch the skin or inject flat, what needle length your pen uses, whether to let the pen warm up from the refrigerator first (many people find a room-temperature injection stings less), and what a normal injection-site reaction looks like versus one worth reporting.