It’s been widely accepted that creatine supplementation always increases body mass.
This notion originated from early research showing that supplementing with creatine (20g/day x 6 days) increased water retention.
Further research then showed that total body water (TBW), extracellular (outside the cell) and intracellular (inside the cell) water increased after only three days of supplementation (Antonio et al, 2021).
Creatine is known as an ‘osmotically active substance’ and therefore, theoretically results in increased water retention….and are perhaps the only negative to supplementation (for some).
Once supplementation starts, creatine is taken up into the muscle. Creatine then attracts water to maintain intracellular osmolality.
I.e. if more solute is found within a solution, the concentration increases. Therefore, to maintain it’s previous (lowered/desired) concentration, more water needs adding to the solution. The same thing happens with creatine and TBW.
Although weight gain via an increased TBW is seen in some studies, not all show the same result.
Some training studies lasting 5-10 weeks have shown no increase in TBW after loading (20g x 7 days) followed by a maintenance dose (5g/day thereafter) (Gann et al, 2016).
Furthermore, males and females who supplemented with a maintenance dose for 6 weeks increased the creatine content of the muscle, enhanced their resistance to fatigue during high intensity exercise…without the increase in TBW (Rawson et al, 2011).
In contrast, Ribeiro et al (2020) showed that creatine supplementation increased TBW by ~7%, of which ~9% was intracellular….which is a good thing if you’re looking to build muscle as this drives muscle hypertrophy over time.
Knowing this, it’s pretty difficult to answer the question as to whether you’ll gain weight or not…ultimately, you probably won’t know until you supplement with it.
If you are an athlete who wishes to experiment with creatine to get the performance gains, but doesn’t want the potential associated weight gain…
I’d recommend that you supplement to find out anyway.
If you do gain the unwanted body mass, simply stop supplementing as the elevated creatine stores within the muscle will return to baseline within ~4 weeks…and will decrease TBW and body mass accordingly. So perhaps try on an offseason?