Transfem

Can and should are two very different things. The two have different absorption rates for different things. You may overdose or it may be totally ineffective. There may be toxic contaminants that taken orally might not be absorbed much, so its relatively safe, but taken rectally might be absorbed into the blood stream. There's also the sanitation issue. The mouth is generally better at capturing and keeping infections out of the body than the rectum, so you may need to sanitize the pills which might be difficult if you don't want them to dissolve. And relatedly, there's the matter of the inactive ingredients. They're designed to be swallowed, but may cause issues left behind in the rectum. Most pills that are swallowed just use starches which isn't a big deal, but to make things dissolve quickly there are other ingredients including sometimes sugars which could encourage microorganism growth if you don't sanitize them or some ingredients may trigger diarrhea or other reactions.

There's usually not research done on a drug taken sublingually as to how it would affect a person rectally, so it's usually not going to be well known.

That said, ask your doctor. If there is any data about doing it, they would be more likely to know.

one potential problem you would likely run into is spiky estrogen blood levels - oral and rectal administration is not ideal for this reason, and is a reason to use routes like injections, patches, or gel.

You ideally want gradual increases and decreases to blood estrogen levels, for so many reasons.

But yes, I think theoretically you will have better absorption from rectal administration because you bypass the liver.

But again this will cause huge and short-lived spikes in blood estrogen making it a terrible idea. Again, the best solution is to change your route of administration to injections, patches, or gel.

There are rectal and vaginal estrogen suppositories that are actually dosed and manufactured for this purpose, I would guess they would have lower doses because of the better absorption. They might be dosed much lower since usually they are supplementation for peri and post menospausal cis women who still produce some endogenous estrogen.

edit: was reading 1 mg estradiol taken rectally will send blood levels over 600 pg/mL at peak and peak is within 3 hours, this sounds like a bad time to me, and I assume oral pills are usually more than 1 mg ...

even my estradiol valerate injections that peak in 1 - 3 days feel too spikey and can create mood swings, and esters like cypionate and enenthate which have much longer half lives feel smoother and more mood-stable when taken at the right dose, frequency, and route of administration.

You will also have better feminization from consistent blood estrogen levels with less spikey methods, whereas spikey oral and rectal administration will leave you without consistently sufficient blood estrogen levels through the day and especially at night when you aren't likely to wake up to dose when you would need to.

Can I ask why you are wondering about this? Are you attempting to get a bigger dose or stretch out your supply or something? If so there are other things that can be done that we know work. I've not heard about rectal E in the DIY community at any point.

If for whatever reason you wish to stretch out your pills/tablets consider this instead: https://stickies.neocities.org/stickies (experimental)

I would recommend just continuing with taking them sublingually without modification unless you have a good reason not to, such as your country making it harder to access hrt.

shell/coating

That's usually an enteric coating - basically protects the medication from your stomach acid so that it can be absorbed in your small intestine.

For that reason, I wouldn't think an enteric coated pill would actually be absorbed rectally - it wouldn't have the acid bath to weaken the coating, and I have no idea how far they normally get into your small intestine before being fully absorbed. With those two factors in mind, my guess is you'd just poop it out eventually, miss your dose, and waste your money.

...also an enteric coated sublingual doesn't make sense - sublinguals don't have a coating, they just dissolve under your tongue.

follow up question, does it need to be jammed in all the way or just past the sphincter muscle?

Only suppositories I'm familiar with only have to be inserted just past the sphincter.

I'd run this by your pharmacist. Easy outcome is the possibility that estrogen is already available as a suppository (I've never heard it personally, but doesn't hurt to ask).

Different routes also often have different dosing. 10mg IV vs 10mg PO are going to hit different - unsure if it's a 1:1 for sublingual vs rectal.

But yeah, I'd take internet strangers' advice (including this post) with a hefty grain of salt on this one, and instead go straight to your pharmacist.