DDAVP nasal spray is available as Adiuretin nasal spray and costs around Rs. 1100/- per bottle. Best way is to use it only at night so that he does not have to get up at night. Avoid during day time. Excess use can lead to water retention and low sodium and low osmolality. Keep a close watch on body weight and swelling of limbs. No investigation is required in general because even if dose is in excess at night, its effect is nullified by no dose during the day. Drink fluids as per thirst which is the best indicator - no thirst means excess dose and excess thirst means less dose is being taken. Investigations, as mentioned by you, may be done 1-2 times a year, if required.