This is what it says on the NHS site:
Thrombocytopenia is a rarer but more serious complication of quinine. It occurs when the number of platelets in your blood falls to a dangerously low level. Platelets help the blood to clot which means people with thrombocytopenia are at increased risk of excessive bleeding.
It is recommended that quinine is only prescribed when:
you have tried the exercise techniques discussed above and they haven't helped prevent your leg cramps
you have frequent leg cramps which affect your quality of life
In these circumstances, you may be prescribed a four-week course of quinine.
After this time, if you have not gained any benefit, the treatment will be withdrawn.