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48 hour vs stat antiemetic regime
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(More concise version of earlier posting above)
EPS is disscussed below, but note that most people don't get EPS.
Some people may get some mild EPS. In very rare cases, EPS could be severe and interfere with / prevent an attempt.
The 48 hour regime may have lower EPS risk since you're not taking so much antiemetic at once.
However, some people find it too "drawn out", and prefer the stat dose. Some people suggest there's a lower vomiting risk with the 48 hour regime, but I haven't seen controlled data to support that.
It's considered a good idea to test taking the antiemetic, eg try 5mg of meto, then try 10mg a day or two later. Ideally, you would build up to testing the actual antiemetic regime you plan to use. You may choose to just do basic tests without testing the full regime.
If initial tests with 5 or 10 mg of meto cause mild EPS, then you might choose to go with the stat dose, so you don't have 48 hours of those symptoms. Taking diphenhydramine (50mg) after the stat dose may help. Another option is to use the 48 hour regime but take 15mg diphenhydramine with each antiemetic dose, to offset EPS. I can't say if this is a good idea since I'm not medically qualified. Testing the actual anti-emetic regime you plan to use is advisable.
If initial tests (5 or 10 mg) don't cause EPS, this suggests a 48 hour regimen is viable, assuming you favour the longer regimen, and the full test run might not be needed. You could still do the full test run, or a partial run, eg 24 hours. If initial tests don't cause EPS, but you're intending to use a stat regime, then a full test run is advisable to see if the stat dose causes EPS.
If you intend to use the stat regime, but did a test run and had significant EPS that wasn't satisfactorily mitigated by taking diphenhydramine, then you could switch to the 48 hour regimen, and re-test. If you started with the intention of using the 48 hour regime but when testing found it to be uncomfortable (eg minor EPS over an extended period), then you might switch to the stat regime combined with 50mg diphenhydramine, and test that.
Another option when experiencing EPS is to switch from metoclopramide to domperidone, which has a very low EPS risk. I can't state whether domperidone is as effective as metoclopramide (further research needed). Also see "Note on Domperidone for SN regimes" below.
Another variation is the 48 hour + stat "combined" regime, where the extra-high stat dose is taken at the last interval of the 48 hour regime. I'm not certain whether that approach offers any benefit. One possible view is that it negates the 48 hour regime lower EPS risk benefit, so it uses a more drawn-out regime, for no apparent benefit. Some may feel there is a benefit to having the antiemetic in the system for longer.
Some may consider the "full test run" over-the-top, or not want to take unnecessary additional meds.
Some may do little or no pre-testing, and simply go with the stat dose, taking the view that if they get EPS, it won't last long before the lethal item is taken. However, if the EPS were severe, it could interfere with or prevent an attempt (although this should be rare). Ultimately, you have to weigh the risks and decide. It is also possible to use the 48 hour regime without any pre-testing.
If you have a limited time window for your attempt, then you might want to pre-test as much as possible, to try to ensure everything will go smoothly during the planned window.
Having diphenhydramine on standby in case you get EPS is a good idea.
Note on Domperidone for SN regimes :
Domperidone is apparently not compatible with H2 blockers such as Tagamet / Zantac. The general opinion seems to be to stick with a basic antacid if using domperidone, eg Rennie, milk of magnesia, etc. Note that antacid is not considered to be essential, so it could be omitted. If using Domperidone you would want to leave a 30 minute gap before taking the antacid (that 30 min gap tends to be part of the SN protocol anyhow if following Stan's guide), since the domperidone leaflet states "Domperidone should be taken before meals and antacids".