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The injection is more potent and will achieve therapeutic levels more quickly, so it is not surprising that injectable sumatriptan is better for some people. Many people do not like to use injectable medicines, and the nasal spray provides a quicker level than does the pill formulation of sumatriptan.
This is not unique to sumatriptan, as most medicines that are available in injectable and other formulations are more potent in the injectable form.
If the nasal spray does not help or is only partially effective, then yes, the injectable formulation is a possibility and it can be prescribed by your GP.
If I can provide any further information, please let me know.