As growth hormone is naturally produced in the human body, the side effects of HGH treatment are rare. Generally, these side effects are caused by taking higher-than-recommended doses of HGH over longer-than-recommended periods of time. However, HGH has been known to occasionally cause:

Hypoglycemia
Hypoglycemia is the opposite of diabetes. In people with diabetes, their bodies do not produce an adequate amount of insulin, leading to dangerously high glucose levels. People with hypoglycemia have too much insulin, leading to too low glucose levels. Because HGH increases the level of insulin in your body, some people have suffered from hypoglycemia after taking HGH drugs.

Acromeglia
Acromeglia is a disease that causes abnormal bone growth. HGH cannot cause acromeglia, but it can speed up the progression of the disease in people predisposed to it.

Extended Belly
When used as a bodybuilding drug, HGH can cause a bodybuilder to have an extended belly if the drug is not used as directed. However, this defect is rare in users who take the drug as directed.

Carpal Tunnel Syndrome
Carpal tunnel syndrome is caused by a pinched nerve in the wrist. It can lead to loss of hand movement. HGH has been known to cause carpal tunnel syndrome in users who do not take the drug as directed.

Joint Pain
Because of its ability to cause growth in a person’s body, some users report joint pain while taking HGH. This side effect is usually temporary.

Source: http://www.somatropin.net/hgh-sideeffects.htm