30mg sounds like a sub-perceptual dose... What is your reason for taking it?
In general taking any dissociative every day has the potential to cause long term negative changes in your brain that may or may not be reversible, there just isn't enough data to say. DXM is also an SRI which I believe may complicate things further but I'm not sure how likely this is to be relevant with such a low dose.
Again, as a general rule, taking any more DXM than you need every day should be avoided, so if 30mg is working for you then I would advise that you do not take any more, assuming that you intend to continue taking it every day.
This dosage to me sounds like something that may have been prescribed, in which case you should discuss this with your doctor. Or if you are self-medicating for some other condition - again, consider going to see a doctor to discuss alternatives. I'd say it's unlikely in the short term that such a dose is going to cause harm, but I would try not to make such daily dosing a long term thing and again I would suggest not increasing your dose if there is no pressing need for you to do so.
I get distinct effects from 30mg, too, although I don't really like them personally.
Do you notice it working for making your codeine more effective? If it is, then you don't need to raise the dose.
It's prescribed in doses around this, combined with another chemical (quinidine) to potentiate it, as Nuedexta - for a rare condition called pseudobulbar affect but also proposed as an antidepressant so - yeah it's pretty safe if you happen to like it. I get effects from small doses too, not 30mg but 50 or 75 and it's a tad too speedy for me but motivating for sure..
Yeah we don't have Nuedexta here, unfortunately as it might save me from quite some trouble to have this prescribable ... For cough, they say the highest therapeutical dose is around of 120mg HBr per day. Yet are there reports of people gone through successful detox of multiple gram daily DXM habits with no sequelae. It's a bit harsh on the gut and dumps loads of NE on your system, leading to tachycardia and restlessness, but nothing too serious.
I was just recently shocked when I thought, let's give the established medicine another try, and got a Rx for sertraline - because the previously used and tolerated venlafaxine didn't cut it. So well. This ugly stuff brought me bona fide auditory hallucinations. Hearing voices. Man, if I wasn't me, they'd just added other toxic antipsychotics on top of that, cause nobody would blame a SSRI when one gets psychotic symptoms and has tolerated other ones before, and did drugs ... so, just my 2 cents, DXM is pretty safe. Just listen to your heart, maybe take a beta blocker if you get tachycardic (that's what I'll do next).
Cause DXM is just unique among all antidepressants in that it actually works.. always.. for me at least, and you might have your own reasons for trying it.
Edit: Why taking the codeine after the DXM? Use it the other way round and you'll avoid the saturation.