Two large analyses show that creatine combined with strength training measurably increases muscle strength and muscle mass, including in non-athletes and older adults. The effect is consistent and causally demonstrated, but limited without strength training. Creatine monohydrate (3-5 grams per day) is safe and inexpensive, with the exception of people with kidney conditions.
Creatine monohydrate is one of the few supplements where the evidence is truly complete: two large analyses, one in adults under fifty and one specifically in older adults, both show that it noticeably increases muscle strength and muscle mass when combined with resistance training. The numbers are concrete: upper-body strength increased by an average of around 4.4 kg, lower-body strength by around 11 kg, compared with a placebo. Not spectacular, but consistent and causally demonstrated.
The key lies in that combination. Without resistance training alongside it, creatine does little; it amplifies the training effect rather than replacing it. The weight gain you may see in the first few weeks is fluid in the muscle cells, not fat.
Safety is not a major concern here. Creatine monohydrate has been intensively studied for decades and is regarded as one of the safest supplements available. The one thing to bear in mind: if you have a kidney condition, discuss it briefly with your doctor. For everyone else, 3 to 5 grams per day of plain creatine monohydrate is the standard approach, widely available and inexpensive. There is no reason to buy more expensive variants.
Strong evidence, based on 2 source(s), including controlled or causal research.