Short answer: The medicine can be absorbed faster and less predictably. Studies of insulin found that injection into muscle sped up absorption, about twice as fast for NPH insulin, and made it more variable from day to day. Expert recommendations say to avoid muscle, especially with long-acting insulin, because severe low blood sugar may result. Ultrasound data show 4 to 5 mm needles at 90 degrees rarely reach muscle in adults.
Two small studies measured this directly with labeled insulin injected into the thigh:
| Insulin | Under the skin | Into muscle |
|---|---|---|
| NPH insulin, at rest | 10.3 hours | 5.3 hours |
| Regular insulin, at rest | 338 minutes | 232 minutes |
| Regular insulin, during exercise | about 290 minutes | about 112 minutes |
For NPH, day-to-day variation was also higher in muscle, and the authors concluded that injecting it into muscle should be avoided. The regular insulin study was planned: it found blood sugar was steadier with mealtime insulin given into muscle under study conditions. It shows that muscle changes timing, not that it is a home technique.
The 2016 FITTER recommendations, written by 183 diabetes experts from 54 countries (Frid 2016), say intramuscular injections should be avoided, especially with long-acting insulins, "because severe hypoglycemia may result." They also say the shortest needles, 4 mm pen needles and 6 mm syringe needles, are safe, effective and less painful as a first choice.
Labels for other subcutaneous medicines say the same in plain terms. The Ozempic Medication Guide says not to inject it into a muscle or vein, and the Xyosted label says to avoid intramuscular injection of its under-the-skin testosterone.
An ultrasound study of 388 US adults with diabetes (Gibney 2010) measured skin and the fat layer at common injection sites:
| Site | Skin | Fat layer under the skin |
|---|---|---|
| Arm | 2.2 mm | 10.8 mm |
| Thigh | 1.9 mm | 10.4 mm |
| Abdomen | 2.2 mm | 13.9 mm |
| Buttocks | 2.4 mm | 15.4 mm |
Skin thickness barely varied between people, but the fat layer did: women had about 5 mm more, and BMI shifted it too. The authors found needles of 8 mm or longer, inserted straight in, may often reach muscle in the arms and legs of men and people with a BMI under 25, while 4 to 5 mm needles reach the fat layer with minimal risk of muscle in nearly all adults.
Gibney 2010 notes that injections into muscle may increase pain, but none of the studies above used how a shot felt to tell the two apart. For insulin, the practical risk is a faster or bigger drop in blood sugar, so the signs of low blood sugar are worth knowing. For other medicines, the prescriber or pharmacist can say whether a dose given too deep matters.
Needle length and site are both choices to set with the care team. Dose Track logs the site for each injection, so a pattern of sore or unusual spots can be spotted over time. Needle sizes are explained on needle gauge and pen needle.
Read more on Dose Track: Sub-Q injection guide · Subcutaneous injection · Intramuscular injection · Needle gauge