Some meds can cause constipation. Better call your doctor and let him/her know.
Can medication stop me from needing to go poo? I haven't been in 6 days.
Yes. It's best to ask your doctor what steps to take to control that issue. It may be as simple as just eating a few plums.
Not going in 6 days can be a real problem. A friend of mine had that problem and it was colon cancer. That might not be the case with you but it certainly deserves attention from a doctor.
Absolutely. Constipation is a side effect to any medication. If you haven't gone for six days, seek the advice of your doctor as he will know the best regimen for you to take/do to get you to go!