My point being, that, and the fact that your avatar appears to be female, are the reason so many guys are viewing your profile.

It only becomes apparent subsequent to viewing your profile that you're a guy.

In the UK, yes. I don't know about the USA.

Also, let's not shout too loudly about it outside of here. If the regulatory authorities discover people are misusing it in order to intentionally gain weight, they may tighten control of it.

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The medication Periactin contains the same active ingredient Cyproheptidine.

☝ This.

Best practice for gaining with double cream:

1. Aim for between 200ml and 500ml per day, gradually increasing quantity over a few weeks.
2. Mix with milk and milkshake flavouring, such as Nesquik to improve palatability.
3. Spread consumption over the day, or you'll overwhelm your digestive system, reducing calorie absorption and making yourself feel unwell.
4. Mix your shake in a sports bottle and keep it with you throughout the day.
5. Eat or drink something sugary each time you drink some of the shake in order to encourage insulin release.
6. Wonder why your clothes are getting so tight.

Rapid gains tend to accumulate first in abdominal fat, before being redistributed around the body over time. It's likely that, if you were to only maintain from here onward, you'd experience a reduction in abdominal fat.

Sorry to pick fault here, as you're clearly trying to help, but you've misunderstood a couple of things:

Firstly, yes, high fat diets can be ketogenic, but only if your carbohydrate intake is very low (20-50 calories maximum per day, depending upon the individual). This is actually a very difficult thing to unintentionally achieve and sustain. It's extremely unlikely that adding fat to a person's diet is going to see them experience a significant reduction in calories from carbohydrates.

In fact, with fats being the most calorie dense macronutrient available (9 kcal per gram, whereas protein and carbohydrate are 4 kcal per gram), they can play a significant role in weight gain. The important thing is to eat them alongside carbs. Carbohydrates play two roles in weight gain: They contain calories, but importantly, in order to maintain consistent blood sugar, they produce an insulin spike causing the body to store the calories, irrespective of their origin, in fat cells.

You only have to read the boards here to see that fats, especially in dairy products such as heavy cream, are very effective in promoting weight gain. The important thing is to use them alongside carbohydrates.

I'd also caution against giving advice that might lead someone to suffer water retention.

Trans fats have been shown to increase belly fat. They're also deadly, unfortunately.

Rapid weight gains very often initially accumulate around the belly, only to be redistributed around the body over time. This very much depends on the individual though.

There are limits to how much and how fast your digestive system can process and absorb any nutrient, fat included. While it's almost certainly possible to drink a litre of double cream in a day, you will find that only a proportion will be absorbed and it'll almost certainly give you a stomach upset.

I recommend you take it slow at first, being careful not to drink so much in one go as to overwhelm your digestive system.