An important bioactive compound of all medicinal mushroom extracts are anti-oxidants, in particular polyphenols and their derivatives. Lactose (mainly found in dairy products) binds polyphenols, potentially neutralizing their anti-oxidant effects.
Research reports show conflicting results, which is why we recommend to either choose lactose-free dairy products or to take dairy products at a different time (at least an hour away) than the supplement. Just to minimize the chances of negative effects. (link)