Is there an international standard for medical prescriptions, you can carry around and show to any pharmacy?

As far as I know, there is not such a world-wide standard, however there is an EU regulation. From www.europe.eu:

A prescription delivered by a doctor in your country is valid in all EU countries. However, medicine prescribed in one country might not be available in another, or it may bear another name.

As of 25 October 2013 you are able to ask for a cross-border prescription which is intended for use in another EU country: these are designed to help the pharmacist understand the prescription easily, the ingredients of the medicine and their dosage.

It's possible a cross-border prescription may be accepted somewhere else too, but there is no guarantee.

In almost all the Middle East countries, almost all prescriptions from other countries will be accepted. The reason behind this is simple, most of the Middle East countries do not have controlled prescriptions. You get a prescription then you go to the pharmacy and get your medicines and the prescription, you can reused it! Pharmacies will give you almost any kind of medicine including antibiotics (except some medicines like Xanax) as long as you have it written somewhere even if its on a piece of paper. I know this is a sad fact but this is how things are here. Also, out of personal experience, I can also confirm the same thing in India, Pakistan, Bangladesh.

In Brazil you can buy many kind of medicines without prescription. Just go to a pharmacy and ask for it.

Exceptions are antibiotics and some restricted ones (like psychotropics), in general those that can cause dependency if used for a long time.

In Brazilian Portuguese, the list for medicines that you'll have to have a medical prescription to buy in Brazil is available at: http://www.ccs.saude.gov.br/visa/publicacoes/arquivos/Medicamentos_controlados_prof.pdf

In addition to @Dirty-flow's answer:

This varies from country to country and from medication to medication. For example:

Dypirone which is available over the counter in some countries is banned in others like the United States. So you will need to look at the prescription and ingredients to know for sure.

It is known that prescriptions from the United States are being filled by pharmacies in Canada and Mexico although not for all drugs. In the US the sale of prescription medications is controlled by FDA and DEA, and all doctors capable of prescribing medications must register with DEA so sales of substances with capacity for illicit use can be managed (somewhat) because of this foreign prescriptions are generally not filled in the US. Canada and Mexico may be an exception for non-controlled substances (Texas' policy as an example).

So if you need a prescription for things like antibiotics in the US the best solution would be to visit an Urgent Care facility which is plentiful and the doctor can give you a prescription fillable in the local pharmacy.

In Canada no foreign prescriptions are allowed. Each province has its own regulations about which prescriptions are considered valid. Until recently, most provinces did not accept out-of-province prescriptions, but the rules have recently been relaxed to allow pharmacists to dispense prescriptions from other provinces in Ontario and some other provinces. You will need to see a local doctor and have the doctor write you a new prescription. A letter in English or French from your current doctor may help expedite the process, but a reputable doctor should still insist on seeing you to evaluate your claims. Doctors have been disciplined or suspended for writing prescriptions without first examining patients.

I think it depends on the country you are coming from and the country you are visiting.

As per @Dirty-flow's answer it looks like prescriptions from within the EU will be honoured, however be careful that your prescription is legible and understandable to the person you are likely to be giving it to. I was in an Airport in the UK and there was a man who "Desperately needed his prescription filled out" but the chemist could not read the name of the drug as it was written in another language so he didn't get his medication.