By meth do you mean methadone? Or methamphetamine? Since you say 30ml I assume you mean methadone, but could you please elaborate? Also I am not clear at all with what you mean by cough? I am injected opiates for many years and have never gotten a cough after shooting up.. That is definitely not a normal reaction. Opiates are a cough suppressant.
But if you are shooting up methadone syrup from the clinic, please do yourself a favor and stop now. There are many other ingredients in it like coloring, sugars, preservatives, etc that are down right horrible to inject.. When you inject things, foreign particles and whatnot can get lodged in the lungs, due to the filtration methods of your body's circulatory system. Over time this can cause pulmonary edema and other complications.
Forgive me if you are talking about something else, though, but your question is not clear. If you could elaborate we will be able to help you a bit better.