Hydrea may refer to: 

Hydra island
A brand name for the medication hydroxycarbamide
These include: 

Severe allergic reactions (rash; hives; itching; difficulty breathing; tightness in the chest; swelling of the mouth, face, lips, or tongue); fainting; fast or slow heartbeat; nausea; new or worsening chest pain; vomiting. Impaired biotransformation of ISDN to its active principle NO (or a NO-related species)
Neurohormonal activation, causing sympathetic activation and release of vasoconstrictors such as endothelin and angiotensin II which counteract the vasodilation induced by ISDN
Plasma volume expansion
The oxidative stress hypothesis (proposed by Munzel et al. in 1995)The last hypothesis might represent a unifying hypothesis, and an ISDN-induced inappropriate production of oxygen free radicals might induce a number of abnormalities which include the ones described above. Furthermore, nitrate tolerance is shown to be associated with vascular abnormalities which have the potential to worsen patients prognosis: these include endothelial and autonomic dysfunction. In the short run, ISDN can cause severe headaches, necessitating analgesic (very rarely up to morphine)  administration for relief of pain, as well as severe hypotension, and, in certain cases, bradycardia. This makes some physicians nervous and should prompt caution when starting nitrate administration. Mechanism of action
Similar to other nitrites and organic nitrates, isosorbide dinitrate is converted to nitric oxide (NO), an active intermediate compound which activates the enzyme guanylate cyclase (atrial natriuretic peptide receptor A). This stimulates the synthesis of cyclic guanosine 3,5-monophosphate (cGMP) which then activates a series of protein kinase-dependent phosphorylations in the smooth muscle cells, eventually resulting in the dephosphorylation of the myosin light chain of the smooth muscle fiber.