There are many prescription drugs that often cause great thirst, particularly if you are taking a high dose. These include:
antidepressants, especially amitriptyline,
steroids, including prednisone, prednisolone, cortisone and hydrocortisone,
HRT and other medications containing oestrogen - like some birth control medication (contraceptives),
some anti-psychotic drugs, including Zyprexa (aka olanzapine) and other psychotropic drugs,
and some anti-epileptic/anticonvulsant drugs, notably valproate (trade name Epilim).
The thirst is because sodium retention/fluid retention/weight gain/salt sensitivity is a harmful side-effect of these drugs. To prevent or reduce the weight gained, you need to cut down on salt and salty food, because the sodium retention has made you sensitive to salt.
prescribed steroids and HRT