Complex sentence: Generally, intramuscular injections are administered by a trained medical professional; however, prescribed self-administered intramuscular injections are becoming more common for patients who require these injections routinely.
Split into simpler sentences: Generally, intramuscular injections are not self-administered, but rather by a trained medical professional. However, prescribed self-administered intramuscular injections are becoming more common for patients who require these injections routinely.