A lot of tablets have lactose in them as a filler. For most people it's not a problem because the amount is small but can affect those severely lactose intolerant.

Fortunately, not all manufactures use the same inactive ingredients so switching brands is sometimes an option. The pharmacy can check the inactive ingredients (usually they have to call the manufacturer) and if it contains lactose order another brand. A good pharmacy makes a huge difference in this instance. I doubt a mail order pharmacy will do these things but for a "clinic" pharmacy this is routine.