Certain drugs can raise or lower basal metabolic rate (BMR) over the long term, that's true. But you can't experience rapid bodyfat gyrations like that unless you are eating to the point of sickness every day, or unless you have something like stage IV cancer or you're taking DNP (a mitochondrial disrupter) - hence your weight change was largely water gain and loss. Having a puffy face would be a give away.
However, basic physiology should make this apparent: a pound of fat is about 3,500 calories - or roughly 150% of an average man's daily calorie intake. To gain 10lbs of fat in a month, you would thus need 35,000 calories over your BMR.
Now since the body doesn't actually absorb calories with perfect efficiency, and digestion consumes a significant amount, and significant amounts gets crapped out, you're probably actually talking about perhaps double that number - an extra 70,000 calories in a month - to gain 10lb of pure fat.
Since an average man's BMR consumes about 2,500 calories per day - 75,000 per month - you would thus need to eat about 145,000 calories over that month to gain 10lbs of pure fat - which works out to nearly 5,000 calories per day, or double the average man's food intake. That is a lot of food, which even bulking bodybuilders sometimes struggle to intentionally consume.
And even if a drug managed to lower your BMR to just 1,500 calories per day, you would still need to eat almost 4,000 calories daily to put on 10lbs of fat in a month.
This explains why most people only gain fat over the long-term - 250+ excess calories per day (an extra chocolate bar or snack) and within a few months the pounds do start to add, although BMR does tend to elevate as well thanks to homeostasis, slowing the gain somewhat.
