Polytechnic students can attempt to enter local Universities that offer a Medical Degree, but there are a few things that you would need to take note of.
Firstly, your GPA must be near perfect (4.0) based off the Indicative Grade Profile (IGP) of NUS Medicine. As NTU Medicine is rather new, only two students that were from Polytechnic have been admitted. Here is the IGP of NTU courses, you will notice that there is no IGP for NTU Medicine.
Secondly, if you are attempting to enter NTU Medicine, you will need to take the BioMedical Admissions Test (BMAT).
Lastly, do take note that your O Level results constitute 20% weightage in the calculation of your results.
As you can probably already tell, Medicine is fiercely competitive in Singapore, and attempting to gain entry into a local university is rigorous.
If you have deep and sincere passion for Medicine, I sincerely believe that you will be able to gain entry into medical schools regardless of what your diploma was. This is assuming that you have met all their requirements and you are able to display your passion during any interviews that come your way.
All the best!