Quando viajo, costumo dar um pulo ao wikitravel. Tal como com a wikipedia, não se pode apostar muito na - vamos dizer - exactidão da informação, mas pode-se sempre extrair alguma informação util.
Por curiosidade, fui ver a página de Lisboa e eis o que dizia na secção Stay Safe:
There are some episodes of violent crimes (i.e. robberies) and some drug related crimes in places like Bairro Alto and Santos, especially at night. Chances are you'll be approached at least a few times by certain types offering 'hash' or 'chocolate'. Many of the people selling hash, cocaine, or marijuana are actually not offering that at all but rather a mixture of herbs they try to pass off to you as a drug. Oregano, or different types of tea can look similar to marijuana and a mixture of ginger and other ingredients are used to make an ºuvaº or an egg of what appears to be hashish. Cocaine is also occasionally offered by these people, but it is almost certain you will be buying a bag of baking soda if you choose to purchase. Don't let this completely discourage you but just remember that the cops in Lisbon largely do not care about the non-violent drug trade and trafficking. If possible go in a group, be patient, take your time and examine the product well, and you should be alright. Remember its OK to walk away from someone trying to sell you a false or bad product.
Curioso que se dedique tanto a informar o possível comprador de estupefacientes :p
ps: a secção diz mais um par de coisas, mas nitidamente devia ter pelo menos um aviso enorme: If you travel in any of those old yellow trains put your wallet in your front pocket and your hand over it!