To barf, or not to barf: Drugs that control vomiting

Your brain is almost entirely closed off from the rest of your body by a layer of tightly linked cells called the blood brain barrier (often abbreviated the BBB). Although nothing can pass between the cells of this layer, small and/or lipid-soluble chemicals such as water and THC are able to pass through the cells, and thus access your brain. There are a limited number of sites in the brain where the BBB is intentionally circumvented, for here lie neural structures that require access to the general circulation to carry out their functions.

One of these sites is called the area postrema. It is a small tongue-shaped area that lies on the floor of the fourth ventricle in the brainstem, and is involved the co-ordination of emesis, or vomiting. It is also called the chemical trigger zone (CTZ), which references its ability to stimulate other parts of the brain (the nucleus tractus solitarius and dorsal motor nucleus of the vagus - more info here) to cause you to puke if it detects a toxic substance in your blood. This provides the us with a built-in mechanism by which we can potentially get rid of substances that have the potential to do us harm. Trauma, bad reactions to drugs, and certain motions are capable of hijacking the vomiting reflex, which is why they often make you ralph.

Emetics are drugs that stimulate vomiting. They are currently used to help remove ingested poisons from dogs and cats and abused as a dangerous method of weight loss. Historically, emetics were used to induce vomiting in humans as a means of dealing with poisonings by ingestion, but this approach has since been replaced by the use of activated charcoal and/or whole bowel irrigation. Antiemetics are drugs that suppress nausea and vomiting. This is their primary application, and it's a damn good one. Many of the drugs belonging to these classes act via dopamine receptors in the area postrema or the other parts of the brain involved in the vomiting reflex to do their thing.

Some emetics and antiemetics:


  • potent emetic that was used in the past in humans to induce spewing
    • currently only used for this purpose in dogs that have ingested toxic and/or foreign substances
  • nonselective dopamine agonist
  • activates dopamine receptors in the striatum, enabling its use to treat Parkinson's disease
    • Parkinson's is the result of the loss of dopamine-secreting cells such that there is insufficient dopamine to activate receptors in the striatum as part of the normal control of movements by the brain
  • derivative of morphine (a common side effect of opioid use is nausea and vomiting)
  • appears clear in solution but stains things green!

Domperidone (Motilium) and Metoclopramide (Maxolon, Reglan, and Degan)

  • antiemetic dopamine receptor antagonists (block dopamine receptors in the area postrema)
  • in addition to arresting upchucking, they are used to treat gastroparesis
  • metoclopramide crosses the BBB but domperidone does not
    • metoclopramide can worsen the symptoms of Parkinson's disease

4 chemically inspired comments:

Ursa said...

Hmm. Interesting.

We use Reglan (metoclopramide) on patients who've had abdominal surgery of some sort, i believe, in combination with docusate and/or sennokot to get their bowels moving again. I hadn't realized it was an antiemetic, although that makes sense--the faster your stomach empties, the less chance it has to decide that it doesn't want its contents anymore.

We tend to use Phenergan (promethazine) as our drug of choice for nausea. (I think there's another drug we use, but it's not coming to me at the moment. Regardless, we use Phenergan a lot.)

One interesting thing to be aware of with antiemetics is that they can mask the ototoxic effects of drugs like vancomycin and gentomycin, as well as aspirin. (Prentice Hall Nurse's Drug Guide, 2005 ed.)

CND said...

Hey, thanks for the inside scoop. Now I've got to figure out exactly why antiemetics mask otoxocity. It always surprises me what strange and wonderful side effects a drug can produce.

I sort of rushed this post, so I plan on revamping it a bit and adding a couple more drugs at some point in the future.

alek said...

and i always thought BBB stood for
bullshit baffles brains.
Applicable in this instance also.
but this is not a snide comment, just an afterthought

Ursa said...

Woot!1 I have a scoop!1 *grin*

I know part of the danger with antiemetics-plus-ototoxic drugs is that they mask the nausea and vomiting that often show up. But i do know that they specifically mask the ototoxicity, too. I'm not sure why they could keep the brain from noticing the cranial nerve damage. Hrm. I'll try to see what i can find.