Can I take Viagra 25mg once a daily?

Viagra is a prescription medication only, you must take it as prescribed by your Dr., the reason being it carries serious risks not to be taken lightly, so go to the Dr. and get checked by a medical professional.
If you have already been to the Dr. and got your prescription follow it as I mentioned.
If you have any questions regarding this prescription medication or need a dosage adjustment I strongly advice you to contact your Dr. and clarify your doubts.

Warnings
Some medicines can cause unwanted or dangerous effects when used with Viagra. Tell your doctor about all your current medicines, especially riociguat (Adempas) and nitrates.

Do not take Viagra if you are also using a nitrate drug for chest pain or heart problems, including nitroglycerin, isosorbide dinitrate, isosorbide mononitrate, and some recreational drugs such as "poppers". Taking sildenafil with a nitrate medicine can cause a sudden and serious decrease in blood pressure.

Contact your doctor or seek emergency medical attention if your erection is painful or lasts longer than 4 hours. A prolonged erection (priapism) can damage the penis.

Stop using Viagra and get emergency medical help if you have sudden vision loss.

Before taking this medicine
You should not use Viagra if you are allergic to sildenafil, or:

if you take other medicines to treat pulmonary arterial hypertension, such as riociguat (Adempas).

if you take nitrates.

Do not take Viagra if you are also using a nitrate drug for chest pain or heart problems. This includes nitroglycerin, isosorbide dinitrate, and isosorbide mononitrate. Nitrates are also found in some recreational drugs such as amyl nitrate or nitrite ("poppers"). Taking sildenafil with a nitrate medicine can cause a sudden and serious decrease in blood pressure.

To make sure Viagra is safe for you, tell your doctor if you have ever had:

heart disease or heart rhythm problems, coronary artery disease;

a heart attack, stroke, or congestive heart failure;

high or low blood pressure;

liver or kidney disease;

a blood cell disorder such as sickle cell anemia, multiple myeloma, or leukemia;

a bleeding disorder such as hemophilia;

a stomach ulcer;

retinitis pigmentosa (an inherited condition of the eye);

a physical deformity of the penis (such as Peyronie's disease); or

if you have been told you should not have sexual intercourse for health reasons.

Viagra can decrease blood flow to the optic nerve of the eye, causing sudden vision loss. This has occurred in a small number of people taking sildenafil, most of whom also had heart disease, diabetes, high blood pressure, high cholesterol, or certain pre-existing eye problems, and in those who smoked or were over 50 years old. It is not clear whether sildenafil is the actual cause of vision loss.

Viagra is not expected to harm an unborn baby. Tell your doctor if you are pregnant or plan to become pregnant.

It is not known whether sildenafil passes into breast milk or if it could harm a nursing baby. Tell your doctor if you are breast-feeding a baby.

Do not give this medicine to anyone under 18 years old without medical advice.

Source: https://www.drugs.com/viagra.html