Translation of "can"

Question

I know can is poder in Spanish. But generally it would indicate the ability to do something rather than the permission.

You cannot drink.

No puedo beber.

The above sentence could imply: 1) that the subject is unable to drink, and 2) that the subject is forbidden to drink.

Does poder cover both the scenarios? Or is there a different word I should consider using to imply forbiddance?

The doctor says you cannot (are forbidden to) drink either milk or juice.

For the above sentence, would the following be the translation preferred by a native speaker?

Dice el doctor que no puedes tomar ni leche ni jugo.

If not, what would be the most natural way to convey the sense?

P.S. If there are regional quirks, I'd prefer Mexico-centric answers.

Answer 1

In Spain you can use poder for both cases, it's OK. But, if you want to specify that you cannot do something because is forbidden, you can use no tienes permitido ... or tienes prohibido ... clauses. For example:

El doctor dice que no puedes beber ni zumo ni leche.

El doctor dice que no tienes permitido beber ni zumo ni leche.

El doctor dice que tienes prohibido beber zumo y leche.

Answer 2

In addidion to itziki answer, to express the fact you are unable to do something not because it's forbidden but for some other reason, you might use ser capaz, eg:

No soy capaz de beber leche por que me da nauseas.

Answer 3

To wrap up the answers given by itzike and jlliagre, you can interpret the verbs as follow:

No puedes beber leche = you cannot drink milk

No tienes permitido beber leche = you are not allowed to drink milk

Tienes prohibido beber leche = you are forbidden to drink milk

No eres capaz de beber leche = you are not able to drink milk

Answer 4

There is also the passive se method of saying, "You can't do that."

For instance,

Voy a robar el banco

No se lo permite!

Translating to "It's not allowed" or "You can not do that!"