Common side effects include tiredness, loss of appetite, diarrhea, abdominal pain, hiccups, itchiness, pneumonia, and blood pressure changes.

In addition to this it also plays a key part in the transmission of pain impulses from the peripheral receptors to the central nervous system.

Aprepitant has been shown to inhibit both the acute and delayed emesis induced by cytotoxic chemotherapeutic drugs by blocking substance P landing on receptors in the brain's neurons.

[10] It has also been shown to increase the activity of the 5-HT3 receptor antagonist ondansetron and the corticosteroid dexamethasone, which are also used to prevent nausea and vomiting caused by chemotherapy.

This original synthesis was deemed to be workable and proved to be a crucial step in achieving commercialization; however, Merck decided that the process was not environmentally sustainable.

This was due to the original synthesis requiring six steps, many of which needed dangerous chemicals such as sodium cyanide, dimethyltitanocene, and gaseous ammonia.

[15] The gamble of taking the drug out of clinical trials proved to be successful when shortly afterwards the team of Merck researchers came up with an alternative and more environmentally friendly synthesis of aprepitant.

In addition, the new process also reduces the amount of solvent and reagents required by about 80% and saving an estimated 340,000L per ton of aprepitant produced.

[15] The improvements in the synthesis process have also decreased the long-term detriment to the natural environment associated with the original procedure, due to eliminating the use of several hazardous chemicals.

[19][20][21] Beyond suggestions that PET receptor occupancy must not be used routinely to cap dosing for new medical indications for this class,[22] or that > 99% human receptor occupancy might be required for consistent psycho-pharmacological or other therapeutic effects,[21] critical scientific dissection and debate of the above data might be needed to enable aprepitant, and the class of NK1 antagonists as a whole, to fulfill preclinically predicted utilities beyond chemotherapy-induced nausea and vomiting (i.e., for other psychiatric disorders, addictions, neuropathic pain, migraine, osteoarthritis, overactive bladder, inflammatory bowel disease and other disorders with suspected inflammatory or immunological components.

However, most data remain proprietary and thus reviews on the expanded clinical potential for drugs like aprepitant range from optimistic[23] to poor.

[24] Aprepitant has been identified as having strong potential in treating protracted vomiting episodes in individuals with cannabinoid hyperemesis syndrome.

[25] This syndrome is characterized by nausea, cyclical vomiting, and cramping abdominal pain resulting from prolonged, frequent cannabis use.