The various gas microbubble contrast media are generally safe with low toxicity in humans. The tolerance of these agents is much higher than that of most x-ray agents, a reflection perhaps of the higher expectation of safety and convenience for ultrasound.
Extensive preclinical and clinical trials have demonstrated an excellent ultrasound contrast agent safety profile, the main side effect being a mild and transient local discomfort at the injection site which results from the high osmolality of these agents. Each contrast agent has its own profile of adverse effects, but all have been trivial thus far.

See also Ultrasonic Contrast Agents.

Any abnormal reaction of a patient to an examination or procedure, like for example side effects of contrast agents or claustrophobia. Claustrophobic attacks as can happen with MRI are unknown with ultrasound examinations. Adverse reactions with ultrasonic contrast agents are very infrequent. In general, adverse reactions increase with the quantity of contrast media and also with the osmolarity of the compound.
Most frequently encountered adverse reactions are: Heat sensation, dizziness, nausea, hypotension due to vasodilatation, which can progress to hypotensive shock and anaphylactic reactions.

Cavitation is any activity of highly compressible transient or stable microbubbles of gas and/or vapour, generated by ultrasonic power in the propagation medium. Cavitation can be described as inertial or non-inertial. Inertial cavitation has the most potential to damage tissue and occurs when a gas-filled cavity grows, during pressure rarefaction of the ultrasound pulse, and contracts, during the compression phase. Collapses of bubbles can generate local high temperatures and pressures. Transient cavitation can cause tissue damage.
The threshold for cavitation is high and does not occur at current levels of diagnostic ultrasound. The introduction of contrast agents leads to the formation of microbubbles that potentially provide gas nuclei for cavitation. The use of contrast agents can lower the threshold at which cavitation occurs.

Types of cavitation: