The many prescription drugs that result in salt sensitivity don't just cause morbid obesity, they damage every system in your body: heart, kidneys, liver, skin, blood vessels, metabolism, etc.
Drugs like tricyclic antidepressants, and like HRT and corticosteroids, and many other drugs, including anti-psychotics and anti-convulsants, cause relaxation of the muscles of the blood vessel walls, with sodium retention as a side-effect. This in turn leads to water retention because extra sodium/salt attracts water to itself. The relaxed state of the blood vessel walls and the impaired kidney function permit a greater blood volume, i.e. permit the incursion of more salt and its accompanying water. If the drugs are taken for long enough, the extra fluid in the body (salt water/fluid retention in the veins = extra blood volume) results both in higher blood pressure (because of the extra pressure on the walls of the blood vessels) and, obviously, in weight gain from the extra volume of blood. The blood vessel walls become weakened and the kidney function impaired by the extra blood volume to have to deal with. A general term for this sort of problem (not always caused by prescription drugs) is salt sensitivity. Since more and more drugs are being prescribed by the drug-oriented medical profession, more and more people are becoming sensitive to salt, and therefore gaining weight, especially if they eat a lot of processed food, well-known to contain a lot of added salt. (I’ve never understood or agreed with the usual claim that relaxation of the blood vessels lowers blood pressure. Maybe this is with people who are not sensitive to salt. – I don’t know.)
Extract from http://www.mayoclinic.com/health/sodium/NU00284
“Your kidneys naturally balance the amount of sodium stored in your body for optimal health. When your sodium levels are low, your kidneys essentially hold on to the sodium. When sodium levels are high, your kidneys excrete the excess in urine.
But if for some reason your kidneys can’t eliminate enough sodium, the sodium starts to accumulate in your blood. Because sodium attracts and holds water, your blood volume increases. Increased blood volume makes your heart work harder to move more blood through your blood vessels, which increases pressure in your arteries. Such diseases as congestive heart failure, cirrhosis and chronic kidney disease can make it hard for your kidneys to keep sodium levels balanced.
Some people’s bodies are more sensitive to the effects of sodium than are others. If you’re sodium sensitive, you retain sodium more easily, leading to fluid retention and increased blood pressure. The extra sodium can even lead to high blood pressure, which can lead to heart disease, stroke, kidney disease and congestive heart failure.”
Although the Mayo Clinic mentions arteries rather than veins, people who are extremely sensitive to salt, like me, are far more aware of swollen veins and the problems and pain they cause, since the changes are very obvious and visible, rather than problems with arteries, even if these are possibly more dangerous than the over-stretched, fragile, agonisingly painful veins.