Once a month is a little restrictive imo. I'd only start being concerned if it was a weekly habit.

Everyone's different. I share a bowl with my wife every night while we are winding down before bed. Haven't had any negative side effects.

When I was in my twenties, I was a much heavier smoker. I was deeply unhappy with my circumstances, and used it as a way to shut my brain off after work or on weekends. The worst side effect there was hanging out with people I shouldn't have, and spending too much money on it. Now, it's cheap and everywhere.

I would say just don't do it more than once a month, assuming you're referring to weed-adjacent stuff that mellows you out but doesn't get you high like the CBD-infused-everything-under-the-sun products. Idk it's a shifty thing. Not addictive in a chemical sense but all too easy to integrate into your daily life. Then your baseline resets and non-medicated existence seems anxiety-inducing and dull.