General

Is there any pill/substance that doesn’t require a prescription I can take to boost weight gain? I’ve looked all over the Internet and haven’t found much - does anyone have experience with this? Thanks!

I have read that Benadryl taken for prolonged periods of time cause a decent amount of weight gain.

Nah I've been taking Benadryl as a sleep aid for years. No noticeable weight gain. Your best bet would be weight gain powder that you can find at most pharmacies/supplement stores

There isn't a substance in that way that simply causes weight gain and nothing else.

Your best bet is to get calorie and nutrient dense supplements like protein powder, bars, ensure, etc.

It’s not over the counter but I’m on mood stabilizers and they cause very rapid weight gain.

As a general rule of thumb, please do not take prescription medications to gain weight. You may get fat, but you'll have other issues too.

There's strong evidence showing that consuming maltodextrin can cause significant and rapid weight gain, but it can also cause very high insulin spikes which could lead to insulin resistance and diabetes so it should be used with caution. It's available over the counter and not too expensive.