Common (>10% users in published RCTs / GHD replacement and anti-aging cohorts):

• Edema / fluid retention (44% in Liu 2007 healthy elderly; lower at GHD replacement; higher at cosmetic doses). Peripheral, sometimes facial.
• Arthralgia / joint pain (28% in Liu 2007 healthy elderly).
• Headache (especially first weeks).
• Injection-site reactions (mild redness, lipoatrophy with poor site rotation).
• Tingling / numbness in extremities (paresthesia from fluid retention near nerves).

Less common (1-10%):

• Carpal tunnel syndrome (24% in Liu 2007 healthy elderly; 5-10% in GHD replacement) — median nerve compression from soft-tissue edema. Often dose-limiting.
• Fasting glucose elevation (5-20 mg/dL; bigger in pre-diabetic users).
• HbA1c elevation (0.1-0.3 over 6+ months at replacement; larger at cosmetic doses).
• Mild blood pressure elevation (fluid expansion + sodium retention).
• RHR increase (mild).
• Lipid panel shifts (improvement at GHD replacement; mixed at cosmetic doses; sometimes triglycerides ↑).
• Gynecomastia in men (18% in Liu 2007 with HGH ± sex steroids — uncommon at GHD replacement; more common at higher doses).
• Subclinical hypothyroidism (GH alters T4→T3 conversion; new-onset or worsening of latent hypothyroidism in 36-47% of GHD adults per literature).

Rare-serious (<1% but worth knowing):

• Insulin resistance / new-onset T2DM in genetically susceptible users on chronic high-dose protocols. Direct (GH antagonizes insulin) plus IGF-1-mediated. Acromegaly literature supports 2-3× increased diabetes incidence at sustained supraphysiologic GH.
• Cardiac hypertrophy / cardiomyopathy at chronic high-dose use. LV wall thickening documented in long-term high-dose bodybuilding case series. Acromegaly cardiomyopathy is the worst-case end-stage.
• Theoretical / epidemiologic IGF-1-mediated cancer concern. Strongest argument against chronic supraphysiologic GH/IGF-1 in the GH-axis class. Acromegaly patients have 2-4× increased colorectal cancer; epidemiology links upper-quartile IGF-1 with prostate/breast/colorectal cancer; Laron syndrome (genetic IGF-1 deficiency) has near-zero cancer; pediatric GHD registries (KIGS, SAGhE) modest second-neoplasm signal in childhood-cancer survivors but not in idiopathic GHD; adult GHD replacement (KIMS) clean. At physiologic replacement risk likely small; at chronic supraphysiologic doses with IGF-1 above upper-normal range for years, risk model becomes more concerning.
• Acromegaloid changes at chronic high-dose use: jaw growth, frontal bossing, hand/foot enlargement, organomegaly. Bone changes essentially irreversible; soft tissue partially reversible.
• Slipped capital femoral epiphysis in children with open epiphyses — pediatric risk.
• Idiopathic intracranial hypertension — papilledema, headaches, vision changes; rare; usually reversible.
• Sudden death in PWS with severe obesity / sleep apnea — black-box-style warning.
• Pancreatitis — rare; described in long-term high-dose users.
• Hypersensitivity / anti-GH antibody formation — rare with modern recombinant somatropin; was more common with original Protropin (methionyl-GH).

Specific watch periods:

• Weeks 1-4 — peak edema, headache, paresthesia. Most users discontinue here if they're going to.
• Weeks 6-12 — peak IGF-1; check labs. Reduce dose if IGF-1 above upper-normal.
• Months 3-6 — cumulative metabolic shifts (HbA1c, lipid panel) detectable; re-evaluate.
• Months 6-24 — long-term territory; cancer / cardiac signal monitoring.
• Years 5+ at chronic high-dose — acromegaloid changes emerging.