Some men can watch in horror as their todger withers by up to an inch, for a variety of reasons.
There is no normal size or shape when it comes to the penis, everyone is different.
On average an erect penis measures about 5.2 inches long with a girth of 4.6 inches, according to research from BJU International.
A flaccid penis measures about 3.6 inches long on average, with a girth of 3.7 inches.
But no matter where your member sits on the scale, there are a few factors that can cause it to drop a few sizes.
1. Exercise
Turns out working out might help you bulk up in all areas but below the belt.
Dr Richard Honaker, chief medical officer at Your Doctors Online warned exercise can make a bloke's todger shrink.
He told Fatherly: "It all has to do with blood flow.
"When working out, the body sends blood to the muscles and joints, thus reducing the amount to the penis and scrotum.
"There is nothing to worry about and nothing to do to prevent it."
The good news? The shrinking effect in this case, is only temporary - and so shouldn't put blokes off getting fitter and healthier.
In fact, not exercising can lead to weight gain which is the next reason you might notice your penis disappearing before your very eyes.
Before we look at obesity and the wasting willy, there's one more word of caution when it comes to exercise.
Blokes tempted to fast-track their bulk, by taking steroids should be warned, they can have a disastrous effect on your todger.
The drugs cause a surge of testosterone, throwing the body's hormone levels off kilter.
As a result, the body shuts down it's natural production of testosterone, which causes your balls to shrink.
Quit the steroids though, and you'll be relived to hear your nuts can return to normal.
One other major side effect of steroid use, is erectile dysfunction - leaving men bereft in the bedroom.
2. Weight gain
So while killing it at the gym will result in your manhood shrinking temporarily, letting the pounds pile on and on is much more damaging to your dangle.
It's not rocket science... the further your paunch protrudes, the harder it is for your member to get air time.
So not only does obesity increase a man's risk of killer diseases, including heart disease and cancer, it will also take its toll on the size of your penis.
Technically, your penis doesn't actually shrink in this scenario.
But, it ends up looking a lot less impressive.
The reason it looks a little on the shorter side is because the erectile muscles are attached to the abdominal wall.
So when your belly expands it pulls your manhood inward.
More good news though, the damage can be reversed by shedding those extra pounds.
So take the short term hit and get down the gym, for the long-term gain - literally.
3. Smoking
We all know smoking is bad for us, increasing the risk of cancer, heart disease and other killer conditions.
Yet many still find it hard to kick the deadly habit.
Well, what if smoking could shave up to an inch off your manhood?
Smoking will make a bloke's penis look less impressive - and as if that wasn't bad enough, it also increases the risk of erectile dysfunction.
Certain toxins in a cigarette damage the blood vessels in your penis, according to Medical News Today.
Unfortunately, not only does that mean the muscle cells in your member will waste away, it also means that no matter how turned on you are you will struggle to get an erection.
And before you think this is some kind of sick joke, there are scientific studies to back it up.
Experts from Boston University of Medicine examined the erect penises of 200 men and found that smokers measured up shorter than those who steer clear of fags.
The 1998 study concluded that smoking inhibits blood flow to your private bits, which can stop your penis from stretching and therefore make it shrink.
4. Ageing
Ageing is hard enough to swallow. But to add insult to injury, a man's advancing years will prove dire for his length down there.
The old meat and two veg will get smaller, as fatty deposits build up in the arteries and reduce blood flow.
This can cause the muscle cells in the erectile tissue to degenerate and wither away.
And it increase the risk of erectile dysfunction.
Another cause can be a build up of scar tissue, from years of small injuries during sex or sports.
The scar tissue can also affect the erectile tissue and cause shrinkage.
5. Medication
All medication comes with unwanted side effects, but some measure up worse than others.
Drugs like Adderall, often prescribed for attention deficit disorders, and some antidepressants and antipsychotics.
Some drugs used to treat prostate problems can also cause your penis to shrink.
A 2012 study into the drug finasteride, used to treat an enlarged prostate, found that men suffered from shrinkage and reduced sensation down there.
Another study, published in the journal Urology, found that 41 per cent of men taking the drug dutasteride, also used to treat an enlarged prostate, suffered sexual dysfunction.
6. Prostate surgery
On the subject of prostates, men who have had surgery to remove the gland due to cancer may experience a loss of size.
In fact, it happens in about 70 per cent of men who have had this surgery.
Experts don't know exactly what causes this to happen, but one theory is that a man may experience muscle contraction after the surgery that pull their member farther into their body.
Another theory is that sexual dysfunction after surgery prevents blood from flowing easily to the penis.
As discussed above a lack of blood flow to the erectile tissue can cause muscle wastage and shrinkage.
Men who experience this may lose anywhere between half to three quarters of an inch - which any man will tell you is a lot.
7. Peyronie's disease
Some men experience a condition called Peyronie's disease, which causes the penis to become curved when it is erect.
It is perfectly normal for a penis to have a slight curve when it is standing to attention, but men with Peyronie's disease will have a prominent curve that causes pain.