Is it haraam to drink Pepsi because it is harmful to the body?

It is not permissible to consume anything that is proven to be harmful to the body, because Allah, may He be exalted, says (interpretation of the meaning):

{And do not kill yourselves [or one another]. Indeed, Allah is to you ever Merciful } [an-Nisa’ 4:29].

And the Prophet (blessings and peace of Allah be upon him) said: “There should be neither harm nor reciprocating harm.” Narrated by Ahmad and Ibn Maajah (2341); classed as saheeh by al-Albaani in Saheeh Ibn Maajah.

If it is proven that some kind of drink or food causes real harm to the body, it is not permissible to consume it. But if it is simply the matter of speculation or a claim, then it is not sufficient evidence to rule that such things are prohibited.

The basic principle with regard to food and drink is that they are permissible, because Allah, may He be exalted, says (interpretation of the meaning):

{It is He who created for you all of that which is on the earth} [al-Baqarah 2:29].

So it is not permissible to say that any food or drink is haraam without sound evidence to that effect, not mere speculation and doubts.

Moreover, if it is proven that these drinks are harmful, and they are deemed to be haraam because of the harm they cause, then it is only haraam to consume the amount that is harmful. If a small amount of it is not harmful, then it is not haraam.

The scholars stated that if something is harmful in large amounts, a small amount of it is permissible.

See: al-Insaaf (10/350); Kashshaaf al-Qinaa‘ (6/189).

If someone wants to refrain from drinking these drinks, as a precaution to protect himself and seeking to remain healthy, there is nothing wrong with that, but he cannot deem it to be haraam except on the basis of clear evidence.

And Allah knows best.