Rapid weight gain may be caused by lifestyle changes like quitting smoking. However, it may also indicate an underlying medical condition, such as kidney disease, insomnia, and cirrhosis.

Many people experience short-term weight fluctuations of 1–2 kilograms (kg) within a 3-day period, which may be caused by lifestyle or dietary changes.

However, rapid weight gain may also be a sign of several underlying health conditions that require medical treatment.

This article discusses 11 possible causes of rapid weight gain and other accompanying symptoms to look out for.

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Certain types of medications may cause people to gain weight, including:

Not every medication in these categories will cause weight fluctuations. However, if a person experiences rapid weight gain, it is important they speak with a doctor before they stop taking them.

Insomnia may contribute to weight gain in children, adolescents, and adults.

A lack of sleep may affect a person’s circadian rhythm, which may lead to weight gain by affecting their:

  • appetite hormones
  • energy needs and expenditure
  • food choices

In a 2015 meta-analysis of 56,000 children, researchers found that those who got less than 10 hours of sleep were 76% more likely to have overweight or obesity, while each additional hour of sleep reduced the likelihood by 21%.

Some people may gain weight initially when they stop smoking tobacco products.

Nicotine, which is found in cigarettes and other tobacco products, may increase a person’s metabolism and reduce their appetite. When a person quits smoking, they may feel more hungry and eat more, causing weight gain.

A 2015 meta-analysis found that the average weight gain in the 5-year period following smoking cessation is 4.1 kg.

However, changes in weight as a result of quitting smoking vary depending on the person. Researchers from another study found that 65% of people who quit smoking maintained or gained less than 5% of their body weight.

Polycystic ovary syndrome (PCOS) is an endocrine system disorder that affects people with ovaries in their reproductive years.

It is characterized by multiple hormonal abnormalities. People with PCOS typically also have insulin resistance, which may result in weight gain.

People with this condition have an increased risk of developing type 2 diabetes and high cholesterol levels.

There is no cure for PCOS, but lifestyle and dietary changes like exercising regularly and eating a healthful diet may help a person lower their weight and improve insulin resistance.

Rapid weight gain or swelling in particular areas of the body (edema) can be due to fluid retention.

According to the American Heart Association, weight gain of more than 2–3 pounds (lb) over 24 hours or 5 lb in a week could be a sign of heart failure.

If blood is flowing slowly to and from the heart, it affects the function of other major organs in the body. As a result, fluid collects in the tissues, which may cause weight gain and swelling in the:

  • stomach area
  • ankles
  • legs
  • feet

A person’s weight usually fluctuates by a few pounds over the course of a day. If their weight returns to normal and they do not have additional symptoms, the temporary increase may simply be caused by bloating and fluid retention.

Other signs and symptoms of heart failure include:

A person should seek medical attention if they experience any of these symptoms alongside rapid weight gain.

Sudden weight gain or swelling in the legs, ankles, or feet may be a sign of kidney disease, such as kidney failure or nephrotic syndrome.

The kidneys are responsible for filtering out waste products and fluids from the body. If they are damaged, they may not properly excrete these and the body may retain fluid, resulting in weight gain.

Other symptoms that may indicate a kidney problem include:

Cirrhosis is a progressive condition in which scar tissue replaces healthy tissue in the liver. This could cause abdominal fluid buildup (ascites), which may cause:

A person should speak with a doctor if they experience rapid weight gain and any of these symptoms.

Hypothyroidism is a condition where a person’s thyroid gland does not produce enough hormones. This may affect many of the body’s functions, such as slowing down the metabolism.

Symptoms of hypothyroidism include:

Cushing’s syndrome occurs when the body produces too much cortisol, which is a hormone that enables the body to respond to stress.

It is rare for a person to develop Cushing’s syndrome without an external trigger. It is more commonly a side effect of taking glucocorticoids, which are drugs that are used to help treat conditions like:

Symptoms of Cushing’s syndrome may include:

If a medication is causing Cushing’s syndrome, a person should speak to a doctor about the possibility of changing their treatment plan.

Acromegaly is a hormonal condition that happens if the pituitary gland produces too much growth hormone.

Symptoms of acromegaly may include:

People with acromegaly may require surgery or radiation therapy to remove a benign tumor on the pituitary gland.

Ovarian cancer refers to any cancer that begins in the ovaries.

Some people may not feel symptoms in the early stages. However, as the disease progresses, people with ovarian cancer may experience weight gain and other symptoms, such as:

  • pain in the abdomen or pelvis
  • difficulty sleeping
  • frequent or urgent need to urinate
  • loss of appetite or feeling full quickly
  • unusual menstrual cycle
  • indigestion

Ovarian cancer often reaches the later stages without detection, so anyone with abnormal pain in the pelvic region should speak to a doctor. Diagnosing cancer at an earlier stage generally results in a better prognosis.

If people are experiencing rapid and unintentional weight gain with no clear cause, they should see a doctor.

The doctor will ask about the individual’s medical history and any additional symptoms. They may carry out a physical examination and blood tests or refer the person to a specialist.

How did I gain 5 pounds in one day?

It is normal for people to experience short-term weight fluctuations. For example, the body may fluctuate between 1–2 kg (2.2–4.4 lb) within 3 days, which may be due to:

  • intestinal contents
  • glycogen stores
  • water stores
  • labile protein stores

Why did I gain 10 pounds in a week?

A person may experience weight fluctuations for a variety of reasons, such as:

A person should speak with a doctor if they experience rapid weight gain that is not caused by the above triggers.

Weight gain and short-term fluctuations may happen for a variety of reasons, such as aging or making lifestyle and dietary changes.

However, fast weight gain can be a sign of an underlying health condition, such as a problem with the thyroid, kidneys, or heart.

Anyone who experiences rapid, unexplained weight gain should see a doctor to determine the underlying cause and develop a treatment plan.