I got pneumonia and doctor prescribed me robitussin ac but not working much. is it ok if i switched to robitussin dm (guaifenesin and dextromethorphan). what is the difference in dm n a?

DM has: dextromethorphan which is a cough suppressant. AC just means there is codeine included, a narcotic suppressant. Pneumonia needs to be treated with antibiotics and expectorant, along with anti-inflammatory medicines. You can take the DM but it is unlikely to be more effective than with codeine. You can try a formulation with Hydrocodone like Tussionex or TussiCaps.