The penny dropped for many after seeing a viral social media post.

The revelation came after an Instagram user by the name of Tired Girl Apparel shared a photo of a t-shirt that said: “You’re so funny!’ Thanks, I can’t have grapefruit.”

The caption added: “If you know…”

It turns out many people didn't know, as the comments section became flooded with people asking what the post was all about.

“Wait am I not supposed to eat grapefruit?” one asked.

Someone else said: “Well this is how I find out.”

Offering up a bit of a clue, a third commented: “I Googled it because I didn’t know it, and I’m on 3 of the meds that say to avoid grapefruit.”

A fourth said: “Me being on Prozac and not knowing this.”

A fifth also pointed out that you ‘can’t have it on birth control pills either’.

Well, it turns out there is some truth to that claim.

Can grapefruit interact with medications?

Grapefruit can interfere with some medications like birth control and antidepressants.

According to Johns Hopkins Medicine, grapefruit can block an enzyme that helps your body break down certain medicines.

“If this enzyme is blocked, too much medication could remain and build up in your bloodstream, causing too-high levels,” their website states.

A few years back, a study in the Canadian Medical Association Journal found that three times the levels of one blood pressure drug called felodipine were reported after patients had a glass of grapefruit juice, compared with a glass of water.

Dr David Bailey, one of the researchers, told the BBC: "One tablet with a glass of grapefruit juice can be like taking five or 10 tablets with a glass of water and people say I don't believe it, but I can show you that scientifically it is sound.

"So you can unintentionally go from a therapeutic level to a toxic level just by consuming grapefruit juice."

The contraceptive pill – especially those that contain oestrogen – is one of the medications that can be impacted by consuming grapefruit.

Birth control app Emme explains how it might increase the effectiveness in a pregnancy prevention sense, but also simultaneously increase the risk of side effects.

"The interaction is primarily due to compounds in grapefruit that inhibit the activity of an enzyme called cytochrome P450 3A4 (CYP3A4) in the small intestine,” it says.

“This enzyme is responsible for metabolizing many substances, including certain hormones found in birth control pills.

“When grapefruit or grapefruit juice is consumed, it can inhibit the function of CYP3A4, leading to a slower breakdown of estrogen. As a result, estrogen levels in the blood may increase.”

Anti-depressants, statins, and antihistamines can also be negatively affected when combined with grapefruit.

Even a small amount of grapefruit can cause severe side effects when taking certain medications.

Your medication may be marked with a grapefruit interaction warning, but always consult your doctor or pharmacist if you're concerned.