Short answer: A needle can nick a small blood vessel under the skin, and blood leaking into the tissue shows as a bruise. Labels count it among injection site reactions: in a 1-year Xyosted study, 6.7% of men had injection site bruising. MedlinePlus says blood thinners such as aspirin, warfarin and apixaban make bruising easier. Insulin and tirzepatide instructions say not to inject into skin that is tender, bruised, scaly or hard.
MedlinePlus describes a bruise as skin discoloration from small blood vessels that break and leak into the soft tissue under the skin. It starts pinkish red and tender, turns bluish, then greenish-yellow, and fades back to normal skin color. Bruises can last from days to months, depending on depth.
For first aid, MedlinePlus suggests ice wrapped in a clean towel, for up to 15 minutes each hour, and raising the area above the heart if possible. It says not to try to drain a bruise with a needle.
Labels group bruising with redness, itching, pain and bleeding as injection site reactions:
| Medicine | What the label reports |
|---|---|
| Xyosted (testosterone, under the skin) | 1-year study of 150 men: bruising 6.7%, bleeding at the site 3.3%. 6-month study of 133 men: bleeding at the site 6.0%, bruising 3.8%. Any site reaction across both: 12.7% |
| Zepbound (tirzepatide) | Site reactions, including bruising: 2% on placebo vs 6%, 8% and 8% on 5, 10 and 15 mg |
| Mounjaro (tirzepatide) | Site reactions: 3.2% vs 0.4% on placebo |
The Zepbound label adds that site reactions were more common in people who developed antibodies to tirzepatide (11.3% vs 1%). The Xyosted studies had no placebo group. See injection site reaction for other medicines.
MedlinePlus says people taking a blood thinner such as aspirin, warfarin, dabigatran, rivaroxaban, apixaban or clopidogrel are likely to bruise more easily. Some supplements are studied for the same reason; see fish oil and blood thinners and vitamin E and blood thinners.
Anyone on a blood thinner who injects a medicine can mention it to the prescriber, who knows whether the injection type matters for them.
The Mounjaro, Lantus and Humalog instructions say to rotate sites and not to inject where the skin is tender, bruised, scaly or hard, or into scars or damaged skin. The Xyosted instructions limit shots to the left or right side of the belly, not within 2 inches of the navel, and add red skin and stretch marks to the list, and ask users to avoid tattoos. A rotation plan keeps a fresh bruise out of the next few sites; see why injection site rotation matters.
MedlinePlus says to contact a provider for bruising without any injury, signs of infection around a bruise, or bleeding into the skin that appears suddenly, such as tiny red dots, or that does not go away. Dose Track logs the site of each shot, which shows whether bruises cluster in one spot.
Read more on Dose Track: Injection site reaction · Injection site rotation · Subcutaneous injection guide · Xyosted side effects · Zepbound side effects