Onrex (Ondansetron 8mg) Tablets

Onrex general information

What is Onrex used for?

Vomiting, which is also called emesis, is a defence mechanism that forces the body to eject toxic or harmful substances that have been ingested before they are absorbed.  It usually begins with nausea, that unpleasant sensation when you suddenly break into a cold sweat, your skin pales and your mouth fills with saliva.  The next thing to happen is retching which is your body preparing to expel the stomach contents, followed by contraction of the abdominal muscles and the diaphragm, with the forceful ejection of the stomach contents, which is the vomiting reflex. 

Nausea and vomiting are common side effects of cancer treatment, including chemotherapy (cytotoxic drugs) and radiotherapy (radiation).  It is also a common reaction following surgery using a general anaesthetic.  Onrex tablets 8mg are an anti-nausea medication used to treat and prevent nausea and vomiting caused by chemotherapy and radiotherapy, and to prevent postoperative nausea and vomiting.  Onrex is not effective at preventing nausea and vomiting due to other causes like motion sickness, which are caused by a different mechanism.

How is the vomiting reflex activated?

There are various ways that the vomiting reflex can be triggered, but they all work through an area in the brainstem called the vomiting centre, which is an area in the brain made up of a complex network of nerves that receives signals from several parts of the body.  These include the gastrointestinal tract when chemoreceptors are activated by toxic chemicals or irritants that have been ingested.  The vestibular labyrinths (semi-circular canals) in the ears detect changes in movement, which can cause motion sickness and vertigo.  Other areas of the brain including the cerebral cortex and the chemoreceptor trigger zone (CTZ) in the area postrema, also send messages to the vomiting centre.  The CZT can detect harmful substances in the circulation and in the cerebrospinal fluid of the central nervous system and also receives nerve impulses from the vagus nerve supplying the gastrointestinal system.  These are all known as afferent pathways that can stimulate the vomiting reflex.  Stimulation of any of these afferent pathways results in messages being relayed to the vomiting centre. 

All signals received by the vomiting centre are then transmitted down a common nerve pathway to the diaphragm, the abdominal muscles, the stomach and the oesophagus.  These signals trigger the vomiting response, which is an involuntary reflex and cannot be controlled once it has been triggered. 

Chemotherapy, radiotherapy and anaesthetic induced nausea

Nausea and vomiting associated with chemotherapy, radiotherapy and anaesthesia for surgery, are thought to be related to the release of certain neurotransmitters (chemical messengers used by nerve cells to communicate) that activate the vomiting centre.  Serotonin (also known as 5-hydroxytryptamine or 5-HT) is one of several neurotransmitter mediators involved in nausea and vomiting induced by chemotherapy, radiotherapy and anaesthesia.  Activation of the vomiting centre by serotonin is a common mechanism by which nausea and vomiting are induced by chemotherapy, radiotherapy and anaesthesia.

How does Onrex work?

Onrex tablets 8mg contains ondansetron, an antiemetic that is used to control nausea and vomiting associated with cancer chemotherapy and radiotherapy, also to prevent postoperative nausea and vomiting.

Ondansetron in Onrex tablets 8mg is a potent and highly selective serotonin (5-HT) receptor-antagonist, of the 5-HT₃ subtype that is found mainly in the central nervous system, including the CZT.  These receptors are also found in the gut, where they are involved in transmitting signals between the gut and the brain via the vagus nerve, a mechanism known to be involved in the vomiting reflex.  Chemotherapy, radiotherapy and surgery cause the release of serotonin (5HT) from the cells of the small intestine, which stimulates 5HT₃ receptors that are found on the vagal nerves in the intestines.  This activates the CZT centre, which sends signals to the vomiting centre to activate the vomiting response. 

Ondansetron in Onrex tablets 8mg has a dual effect and works by blocking the 5-HT₃ receptors both in the peripheral nervous system in the intestine and the central nervous system in the CZT of the brain.  This action prevents activation of the vomiting centre and blocks the vomiting reflex, also relieves the feelings of nausea. 

What does Onrex contain?

Onrex tablets 8mg contain the active ingredient ondansetron hydrochloride, an antiemetic used to treat nausea and vomiting caused by cancer chemotherapy and radiotherapy, also to prevent post-operative nausea and vomiting.  They also contain Hypromellose, iron oxide yellow, lactose, magnesium stearate, microcrystalline cellulose, pregelatinised maize starch, purified water, titanium dioxide.

What are the side effects of Onrex?

Most medications have some side effects, but they are not experienced by everyone.  Some side effects are commonly experienced when taking Onrex, but others are not so common, and you should discuss any problems or concerns with your primary care physician. 

Common side effects when taking Onrex include headache, a sense of warming or flushing, constipation, hypotension (low blood pressure), hiccups, arrhythmias (irregular heartbeat), chest pain.

When should Onrex not be used?

Have a talk with your primary healthcare physician before taking Onrex tablets 8mg so that you have a full understanding of what this medicine is for and how to use it.  There are some reasons for not taking a medication; these are called contraindications, and for Onrex you should consider the following before taking Onrex:

• Have you ever had an unusual reaction or an allergy when taking Onrex?
• Have you had a reaction to any other similar drugs (5-HT receptor antagonists)?
• Are you pregnant or breastfeeding?
• Do you have any problems with your liver?
• Have you had any problems with intestinal blockage or serious constipation?
• Do you have a heart condition or a heart rhythm disorder?

What medications interact with Onrex?

Some medicines interact with Onrex tablets 8mg and may affect the way it works, or are affected by Onrex or increase side effects; you should discuss possible interactions with your primary care physician.  These may include the anticonvulsants phenytoin and carbamazepine, the antibiotic rifampicin, the analgesic tramadol. 

If other medications may interact with Onrex, your doctor will discuss these with you. 

How should Onrex be taken and for how long?

You should take your Onrex tablets 8mg swallowed whole, with a glass of water with or without food.  For prevention of nausea and vomiting following chemotherapy or radiotherapy you will be given your Onrex tablets usually between 30 minutes and 2 hours before the start of the treatment, and possibly again 12 hours later.  The dose you take and how often depends on how emetogenic (nausea/vomiting-inducing) your treatment is and your doctor’s recommendation.  You may also be given Onrex twice daily for 5 days after treatment to prevent delayed nausea and vomiting. 

To prevent postoperative nausea and vomiting you will be given your Onrex tablets 8mg one hour before your anaesthetic.

Missed dose of Onrex

If you miss a dose of Onrex take it as soon as you remember, unless it is time to take the next dose, then skip the missed dose.  Do not take a double dose.

How should Onrex be stored?

You should store your Onrex below 25°C in a cool dry place.