Growth hormone therapy can lead to fluid retention, which in turn causes swelling in the extremities, joint pain, and carpal tunnel syndrome. (Some researchers think any increase in lean body mass comes from extra water rather than muscle growth.) It can also produce the symptoms of diabetes. But all of these side effects go away when you stop using the drug.
There's some evidence that HGH can cause cancer in lab rats, but no one has proven that the same risk exists for humans on hormone replacement therapy. Patients with acromegaly—i.e., those who naturally produce too much growth hormone—are at higher risk for certain types of cancer.