Can I get medicine by walking into a pharmacy ?

Yes, you can buy many medicines in a pharmacy, more than you can in most developed countries without needing a prescription.  My advice is not to play doctor with your health by self-prescribing powerful and possibly harmful medicines - several words for doing that in Spanish are autodosis and automedicación.

Here is a link to a list of medicines allowed by Colombia's government along with allowed prices:

https://minsalud.gov.co/salud/MT/Pagina … olado.aspx

@Flowerchildrenee, yes it is very easy to get many medicines in Colombia by walking in to the pharmacy.  FYI, they may not have your brand name, so workaround is to translate the active ingredient to the Spanish name and hen show that name o the pharmacist.  That's how I've been able to find almost every medicine.  In my experience, medicine is much less expensive in Colombia, compared to my home country of the US. 

I concur with the 2 above comments... at least here in the north side of Barranquilla there's a pharmacy literally every couple of blocks (more or less). Also, compared to the USA, medicine here is far less expensive, and far more accessible (that is, fewer meds are "controlled", you can get them without prescriptions).

That said, let me reinforce what @OsageArcher said: Be mindful of self-prescribing powerful or weird meds. Pre-existing knowledge, self-research, and self-confidence are great, but talk to your doctor first... better safe than sorry, y'know?