Most men with erectile dysfunction (ED) have success with Viagra and experience satisfactory results with the little blue pill. However, some men notice that the drug does not work for them, while others experience a gradual decline in the drug’s efficacy.

Dr. S. Adam Ramin, urologic surgeon and medical director of Urology Cancer Specialists, LA, tells Healthline, “Erections occur when there is a net increase of blood flowing into, as compared to blood exiting, the erectile spongy bodies of the penis.”

“Viagra works at the artery level by enhancing the command of the neurotransmitters to cause dilation and increase flow through the arteries,” he adds.

If there are any neurological or vascular issues associated with the erection process, Viagra won’t work, according to Dr. Ramin.

Before taking the small blue pill, it is important to understand a few important things. For instance, you must avoid taking the drug with alcohol, large or fatty meals, and grapefruit juice. More importantly, you should never expect Viagra to work spontaneously, which is why doctors advise the dosage an hour before having sex.

Dr. Ramin says, “If you take Viagra and immediately try to have sex, it won’t work. The most optimal way to take this medication is on an empty stomach, with no alcohol in your system, one hour before sex.”

If Viagra does not work for the first time, make sure you use it in several different scenarios. Dr. Ramin says, “If you’ve taken Viagra in the most optimal situation more than five times and it hasn’t worked, then it may not be the right treatment for you.”

Dr. Myles Spar, National Medical Director at Vault Health, recommends using Viagra at least once when alone masturbating to make sure it is not a relationship issue before you decide the drug is not for you.

Experts say you may need to try the blue pill as many as 10 times before switching to other ED treatment options.

Dr. Spar says that Viagra not working could mean that the issue isn’t vascular — meaning, increasing blood flow isn’t helping. He explains, “This can happen because of neuropathy or other issues.”

Why doesn’t Viagra work for everyone?

Well, it is true that Viagra may not be the most effective treatment option for all men. Some men fail to achieve its effects due to the dosage, the action mechanism, or underlying health conditions.

Here are a few physical and psychological reasons why Viagra may not work for everyone.

Atherosclerotic Heart Disease: In this condition, there is a significant obstruction to the blood flow in the arteries, including the penile artery. “People with atherosclerotic heart disease have arteries that are plugged with cholesterol plaques or have lost their elasticity,” says Dr. Ramin. And in such situations, the arteries will not respond to Viagra due to vasodilatation.

Nerve Damage: Dr. Ramin says people with diabetic neuropathy or people who underwent prostate surgery that cause nerve injury may find that Viagra does not work.

Peyronie’s Disease: Although rare, Peyronie’s disease is a kind of injury to the body of the penis, which can cause scarring. Due to this condition, the erectile body may not be able to accommodate increased blood flow, according to Dr. Ramin.

Mental Health Issues: Anxiety, depression, stress, or relationship issues are some of the psychological causes of ED. Dr. Spar says another reason Viagra does not work for everyone has to do with anxiety and emotional health, making it difficult to get sexually excited from a neurological perspective.

The bottom line is if Viagra does not work for you, there are other things you can try, such as lifestyle changes, consulting with a urologist or sex therapist, among others. The article was originally published in Healthline.