Here are noted adverse effects from a good monograph on creatine from Drugs.com:

http://www.drugs.com/npp/creatine.html

Adverse Reactions

Evidence for long- or short-term adverse reactions from creatine monohydrate supplementation taken in recommended doses has not been found in either prospective or retrospective studies in healthy subjects. 111 , 112 , 113 , 114 , 115

Safety results are similar in studies in patients treated with creatine for clinical conditions. Few adverse reactions have been reported in trials in patients with neurological and muscle disorders, 51 , 107 , 116 , 117 , 118 and no adverse effects were reported in small studies of gyrate atrophy patients receiving supplementation of normal dietary intake (1.5 to 2 g/day) for up to 6 years. 92 , 94

Most reports of adverse reactions remain anecdotal because documentation in well-controlled trials is lacking. 112 Creatine supplementation elevates urinary and serum creatinine, which can increase the creatinine load on the kidneys; healthy kidneys appear to manage short-term creatine loading without compromised function. 49 , 119 However, patients with a history of renal function impairment or diabetes or those taking nephrotoxic agents should avoid concomitant creatine supplementation or be monitored closely if supplementation is necessary. 113 , 114 , 120 , 121 , 122 Healthy individuals should consider regular testing to detect any potential renal function impairment because of unknown decreased compensatory mechanisms. 114

Anecdotal reports of adverse reactions include dehydration, heat-related illnesses, reduced blood volume, electrolyte imbalances, and muscle cramping. 2 , 8 , 114 Intracellular fluid retention in the muscle cell may predispose to dehydration, but studies are lacking; a dehydrated state before exercise decreases renal concentrating ability. 114 , 120 Optimal hydration is recommended during supplementation to reduce risk of hydration-related adverse reactions. 114 , 120

Two randomized controlled trials reported minor GI upset (eg, diarrhea, dizziness, GI pain, nausea) 85 , 123 ; however, there was no overall difference in adverse reactions between creatine and placebo groups. 123

Supplementation studies using creatine for up to 8 weeks have reported minimal or no liver enzyme elevation 19 , 124 , 125 ; however, some concern exists regarding the reversibility of endogenous creatine synthesis suppression after long-term supplementation. 120 Additionally, no evidence for alterations in blood pressure, renal indices, or plasma creatine kinase activity was found in a study in young healthy men and women. 113

One trial reported an increase in muscle pain during high-dose creatine treatment (150 mg/kg body weight) in glycogen storage disease type V 60 but the reason was unknown. 51

A theoretical concern exists regarding the neurological effect of oral creatine supplementation. Three of 32 documented complaints to the US Food and Drug Administration regarding creatine supplementation have involved seizures. 114 , 120 No causal relationship has been established. 120

An increase in body mass is well documented in controlled trials with creatine supplementation, 5 , 18 , 93 which could have negative effects on mass-dependent activities. 120

The safety of creatine in children younger than 18 years of age has not been established. 8 , 126 A theoretical concern exists regarding the extra creatine load placed on developing organs, particularly the kidneys, as well as the effects on muscle and bone junctions in the skeletally immature. 120

Data are limited in elderly patients; 1 study (N = 32) evaluating the effects of creatine ingestion in sedentary and weight-trained older adults (67 to 80 years of age) reported no adverse reactions with a 52-day oral supplementation regimen (20 g/day for 5 days, followed by 3 g/day for 47 days). 49

Notably, in controlled studies, there was no significant change in blood pressure found in treatment with creatine versus placebo.