The effects of diphenhydramine HCl and dimenhydrinate are virtually the same. However, diphenhydramine HCl produces a more marked, rapid effect. Dimenhydrinate was developed to avoid those effects, but still have the H2 effects.
Dimenhydrinate is the 8-chlorotheophyllinate salt of diphenhydramine HCl. You don't have to worry too much about what that means! [if you wanna know: it means there's an additional bunch of atoms attached to each of the diphenhydramine molecules. This bunch of atoms is stripped off when it dissolves in water, so that the effect of the drug is not changed. However, it adds to the weight of the molecule, which means that the drug is less potent.] The only important difference is potency: 50mg of dimenhydrinate and cyclizine HCl is equivalent to 25mg of diphenhydramine HCl and meclizine HCl. It's basically the same drug. Of course, these drugs are more expensive as a sleep aid than as an allergy medicine, though it is the same formulation. Be sure to keep your eyes open if you want the best deal.
Meclizine hydrochloride is much like dimenhydrinate, except that it produces less sleepiness and has a weaker effect.
taken from
http://www.erowid.org/pharms/dimenhydrinate/dimenhydrinate_faq.shtml
which I would read from start to finish.