NB: Stopping or changing SSRIs during pregnancy can lead to a relapse of depression or anxiety, which carries its own risks for both the mother and baby. HG commonly affects mental health. Do not stop your medications without consulting with your doctor.

Firstly, some definitions and background information: 

• Ondansetron is an anti-vomiting medication, which works by blocking the 5-HT3 (serotonin) receptor. 
• SSRI stands for Selective Serotonin Reuptake Inhibitors and are medications used for anxiety, depression and other mental health conditions. Sertraline, citalopram, and fluoxetine are all common types.
• Both ondansetron and SSRIs affect serotonin levels in the body. Because of this, it’s possible there may be an interaction between ondansetron and SSRIs causing serotonin syndrome, a potentially life-threatening condition. A small number of cases of serotonin syndrome in patients given a SSRI with ondansetron (or other 5-HT3-receptor antagonists) have been reported.

Although very serious, serotonin syndrome is very rare. Evidence of an interaction between the SSRIs and 5-HT3-receptor antagonists is not well established. That means the risk of serotonin syndrome caused by taking SSRIs with ondansetron is likely to be low, and taking both ondansetron and an SSRI will not cause serotonin syndrome in most people. 

If you are taking both, you should be aware of the signs and symptoms of serotonin syndrome including agitation, confusion, fast heartbeat, tremor or changes to movement, or diarrhoea. Seek urgent medical attention if you have any of these symptoms while taking both ondansetron and SSRIs, or other medications which affect serotonin.

Some patients may be at higher risk, including those who:

1. Take multiple medicines which act on the serotonin system e.g. anti-migraine drugs 'Triptans', tramadol or monoamine oxidase inhibitors (MAOI)
2. Are on higher doses 
3. Have underlying health conditions
4. Have conditions which affect the electrical signal in the heart including history of prolonged QT, congenital QT, multiple medications which affect QT, those with underlying cardiac conditions or electrolyte abnormalities.

Talk to your doctor or pharmacist about whether you are at higher risk of serotonin syndrome and what your options are. Remember not treating HG has its own risks - with HG there is no risk-free option to choose from. You deserve the best information, and a thorough discussion of the risks and benefits of all your options, including: 

• Continuing to take ondansetron and your usual medications, and be aware of the low risk of serotonin syndrome.
• Switching ondansetron for other HG medications while staying on your SSRI. However as ondansetron is not the first-line medication in Ireland, it would be unusual for you to be prescribed it without trying other medications anyway, and other medications may not control your HG well enough. 
• Switching SSRIs to other mental health medications while staying on ondansetron. Again there may be risks to this, as other mental health medications may not work well enough for you, or have side-effects for you and/or your baby which are higher than SSRIs.