they're different mechanisms but it is still possible to overdo both shots and transdermal.
check out early research, they killed dogs trying to find out if magnesium acted as a laxative when injected
PHYSIOLOGICAL AND PHARMACOLOGICAL STUDIES OF MAGNESIUM SALTS.—II. THE TOXICITY OF INTRAVENOUS INJECTIONS; IN PARTICULAR THE EFFECTS UPON THE CENTRES OF THE MEDULLA OBLONGATA (1906)http://ajplegacy.physiology.org/content ... 87.extract
they use magnesium injections for all kinds of things safely but I don't know what level of injected mag is considered safe, or how the change in serum level upon injection would compare to oral dose response.
whatever the route into your system, it's still only in your blood. once your background levels are depleted, depending how far it's gone it could still take loads of time to restock tissue stores.