I see related questions such as:
But does Superman need to eat? And if so, what happens if he doesn't?
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Again, this will depend on which version of the canon hero you are reading about. The most classic interpretation of the character, the Silver Age Superman also known as the Pre-Crisis Superman, was completely powered by solar energy.
Meaning as long as he was under a yellow sun, he didn't need to do anything that resembled a biological process, no eating, sleeping, breathing or excreting of any kind; he was more like an energy being wearing flesh. He appeared to be for all intents and purposes a being powered completely by his body's catalytic response to solar energy.
Under an orange or lower energy sun, he tired more easily and was capable of being injured. He healed slower as well. Under a red sun, he appeared to be Human in all the ways that mattered.
Of course, he was completely capable of eating food and processing it normally and his invulnerability included the consumption of toxic materials or poisonous foods. He could eat any kind of food with no ill effects as long as the yellow sun provided him with his invulnerability.
He could also breathe in toxic fumes and contain them within his lungs with no ill effects long enough to clear a space of the fumes and fly them into space before releasing them.
This probably meant he would be immune to the effects of toxic drugs or powerful narcotics since his body only seemed to be affected by things native to his Kryptonian physiology.
The physics for why the Silver Age Superman was completely invulnerable, inside and out, were never completely explained (no surprise) so we were forced to accept his invulnerability meant he was proof against most environments and toxins without question. Only under the presence of Kryptonite (of a variety of colors) or the energies of Chaos magic would he be susceptible to any kind of toxin, drug, or biological agent.
Post-Crisis Superman seemed a bit more human and mentions more than once:
He did need to breathe, just not very often and could hold his breath for a very long time, hours or even days if he needed to. (His body must not NEED oxygen at the same rate ours did, supplementing solar energy for whatever metabolic processes needed oxygen. He never notes whether it was painful to hold his breathe for such extended periods. He has worn a spacesuit and used compressed oxygen tablets when forced into space for extended periods.
He notes he needed to sleep for the same reasons we do, to give his mind a chance to rest and process information through dreaming. He talked about going long periods without sleep noting reduced efficiency if he went longer than a month or two. (The clinical safe record for humans is about 11 days.)
Post Crisis Superman was known to eat, mostly with his friends and family, but it was never made clear whether he NEEDED to eat. Since eating is both about repairing tissue and providing energy to the body, both things handled by his solar energy process, it is conceivable, Post Crisis Superman had no reason to have to eat other than enjoying the food and the company.
The Modern DCnU Superman has as yet not revealed whether he needs to eat, sleep or breathe long term. I suspect DC will continue with the tradition of having Superman emphasize the SUPER while minimizing the MAN aspect of the character.
Lois Lane specifically asks Superman about this when she interviews him in the 1978 Superman film.
From the script:
Lois Lane: How old are you?
Superman: Over twenty-one.
LL: I get it. You don't want anyone to know. And how big are you? How tall are you?
S: About six four. Around two twenty-five.
LL: I assume then that the rest of your bodily functions are normal?
S: I beg your pardon?
LL: Well, putting it delicately, do you eat?
S: Yes, I do, when I'm hungry.
LL: You do. Of course, you do.
Kal-el possesses the biology to process food and drink and breathe. But thuthfully, his powers already ignore the laws of physics. Even if he didn't need to eat per se as sunlight energy would be sufficient to metabolize, that's only energy. His body has to synthesize proteins, amino acids, ATP and other organic chemicals that make up his body and that matter has to come from somewhere. Plants don't "Eat" sunlight, they catalyze and convert it into a soluable chemical energy (Sugar) to metabolize or store for later use, they also manufacture chemicals needed for their physical structure (Cellulose and lignin); that requires chemical inputs of Carbon, Hydrogen, Oxygen and Nitrogen and Magnesium and metabolic trace elements like iron, manganese, etc. I would surmise Superman has a finite chemical reservoir as we all do and his body is simply more efficient at recycling nutrients because the energy needed to do it comes directly from an outside source; unlike basic human metabolism where 60% of our metabolic energy is lost as heat.
In Action Comics Vol 1 #800, the post-Crisis version of Clark told Lana Lang that he saves on living costs because he doesn't need to eat. This conversation took place on the day he moved away from Smallville, so he hadn't become Superman yet when he said this.