You will get more from the drug if you take it orally. Snorting it has a lower bioavailability (less is absorbed) and IVing it has a way shorter duration. So you may need a little less to get high by IVing it, but if wears off in half the time compared to taking it orally which means you will just be IVing half the dose twice as often, making it pointless when you consider the risk/reward of engaging in such behavior.
If you still wish to IV it, get micron filters, bacteriostatic water, and DO NOT HEAT the shot. A 5mg dose would be a better starting point than 7.5, and you should still have somebody close by in case you get sick or worse from it.
There's more to it than that, but if won't elaborate further unless you ask again.
How would he even measure 5mg though? 7.5mg is just a quarter of a roxi. I guess you could up dissolve 7.5mg in 30units of water (2.5mg/10units). Then back load 20 units(5mg) of water into another syringe.