Though I'm not a doctor, I've experienced no negative reactions smoking marijuana while taking cold medicine.
Even still, you should always consult your doctor before mixing any drugs you're unsure of. If you're uncomfortable talking to your doctor about this subject; ask what effects your medication will have on your body (increased heart rate, blood pressure up, etc), and compare these to the effects of marijuana. If the effects overlap; you may want to proceed with caution, or simply use one or the other.
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That is a question that should be asked to a doctor. A doctor prescribes pain medication.
Sometimes it is safe and other times it is unsafe your doctor can go over the risks with you and benefits of the pain medication prescribed to you.
Answer stems from the fact you did not name what type of painkillers.