there have been people who diluted their liquid oral methadone for injection using butterflies and multiple large volume syringes
^I'm pretty sure even that can carry serious health risks.
Some "oral" morphine actually
doesn't contain any flavours/sweeteners/dyes/etc and
can be injected, so it's important to know what kind the OP has to determine whether or not it's safe to inject. But I really think the vast majority of them (particularly any prescribed from a pharmacy and not given in a hospital setting) are likely not safe for IV use, especially without micron filtering. If the OP doesn't know the manufacturer or ingredients it is definitely not worth the risk.
For example - these are all US brands:
Paddock Laboratories and Roxane Laboratories 100mg/5ml oral morphine sulfate solutions contain: morphine sulfate, citric acid, edetate disodium, glycerin, sodium benzoate, sorbitol, and water.
Mallinckrodt 100mg/5ml oral morphine sulfate solution contains: morphine sulfate, anhydrous citric acid, edetate disodium, FD&C Green No. 3, glycerin, purified water, raspberry cream flavor, and sodium benzoate; the pH of the solution is adjusted with diluted hydrochloric acid as needed.
Lannett 100mg/5ml oral morphine sulfate solution contains: morphine sulfate, anhydrous citric acid, edetate disodium dihydrate, glycerin, methylparaben, propylparaben, sodium benzoate, sorbitol and purified water.
If daviseyes responds with more info I could provide better advice on whether their formulation can be injected or not and/or what they could do to make it safer for injection (when possible).