You may have been told (or read somewhere) that the reason to drink plenty of water after massage sessions is to "help flush toxins from the body." In some ways, I wish this were the case, for I'd get a full-body cleanse after receiving a massage. In reality, though, this does not happen. Manual therapy does not significantly move toxins from the body, but it does increase local circulation and helps the body in so many other ways.
According to massagetherapy.com, "there are very real toxins that accumulate in the body, notably persistent organic pollutants (POPs) in fatty tissues and heavy metals in skeletal tissues. However, these toxins are too chemically bound to their target tissues to be significantly liberated by the mechanical motions of massage."
So, why do massage therapists remind you to drink plenty of water then? In addition to general hydration being very important, we encourage clients to drink water after a massage to re-hydrate their tissues we have worked so hard to loosen. The kinks and knots and sticky spots in muscle tissue and fascia (connective tissue) will remain supple and loose longer (and take longer to become "sticky" again) if the body is well hydrated. The best example I can think of right now is a bicycle chain. Without lubrication the bicycle chain's resistance makes it harder for the rider to pedal and shift. The same goes with our body. Without proper lubrication, we don't move very well. And that lovely loose feeling we get after a massage fades a bit more quickly than we'd like.
So, drink up! Feel the power of water to replenish your body!