First, where: Pharmacies are scattered all over Paris. During business hours, you are likely to spot one within a few blocks of wherever you find yourself in the city. Just look for the distinctive green cross logo. “Local” pharmacies, targeting residents of the nearby blocks are typically rather small and may not have everything in stock, but there also exist a few larger ones (see below).
Many pharmacists can act as common-sense doctors, so if you simply describe your symptoms, they'll be able to recommend a medication. Quite a few things that would normally require a prescription in the U.S./Canada can be bought off-the-shelf in France.
Prices may vary quite significantly – entering one of the smaller pharmacies off the poshest streets in the 8th arrondissement, for example, you can be excused for thinking that boxes have been mislabeled as they occasionally show prices that are a multiple of what the equivalent would cost in the U.S. or Canada.
This is why, if you are staying for more than a few days, it probably makes sense to take a trip to one of the lower-cost (and larger-stock) pharmacies, such as City-Pharma (26 Rue du Four, 75006 Paris). Incidentally, this pharmacy is based more on the the American “drugstore” model than on the “boutique” model that is the standard elsewhere in Paris.
Showing 2 places of interest:
* Although most pharmacy clerks are helpful, some may have only a very rudimentary knowledge not only of English, but also of their own inventory. Rather than helping you find the closest thing to what you need, they may simply give a perfunctory reply along the lines of “I don’t think we have anything like that, madame/monsieur”.
In about a dozen trips to smaller pharmacies, I came across clerks who didn't know what hydrogen peroxide-based contact lens solution was (a bottle of this mysterious substance was displayed on the shelf directly behind them – but happened to be out of my reach; getting hold of it required some diplomatic skills on my part along with the good ol' pointing of the finger) or have never heard of rubbing alcohol (a reluctant trip to the stockroom finally disabused them of the notion that the weird thing I ended up having to describe as "l'alcool pour la peau" was a figment of my imagination).
Although such incidents are not the norm, they are common enough to warrant a warning. Be prepared to explain and insist.. if you want to get your stuff.