Oestrogen can be taken sublingually (under the tongue), which is when you dissolve a specially manufactured tablet under the tongue, where it is quickly absorbed by the blood vessels. This article covers some important considerations regarding sublingual oestrogen.
Pharmacology
Sublingual absorption is very quick compared to absorption of oral oestrogen (tablets you swallow). Sublingual oestrogen has a very high peak concentration of oestrogen within an hour of administration, followed by a rapid fall in concentration as it's metabolised (Doll et al., 2022).
Oral oestrogen has a peak concentration 8 hours after administration, and a slower fall in concentration. This has implications for dosing, effectiveness, and monitoring.
Dosing
Doses of sublingual oestrogen are usually the same as doses of oral oestrogen, but dose frequency might differ.
Sometimes it's recommended to split the dose of sublingual oestrogen to be taken twice a day. For example, 2mg taken as 1mg in the morning and 1mg in the evening.
Research has found no difference in effectiveness between once-daily and twice-daily administration of sublingual oestrogen (Cortez et al. 2024).
Effectiveness
A study has found that transdermal oestrogen (patches or gel) was more effective at suppressing testosterone levels than sublingual oestrogen (Cortez et al., 2024).
This is probably oestrogen is absorbed continuously with transdermal administration. This seems to be more effective at suppressing testosterone. Sublingual oestrogen is absorbed very quickly, so might not suppress testosterone as effectively.
If this is an issue, consider looking into testosterone blockers.
Monitoring
Blood monitoring of oestrogen levels need to be timed more carefully when you take sublingual oestrogen. This is because oestrogen concentrations vary more through the day with sublingual oestrogen than with oral or transdermal oestrogen.
It's best to wait at least 4 hours after taking sublingual oestrogen dose before doing a blood test. This helps you avoid misleadingly high results from the initial peak.
References
Cortez, S., Moog, D., Lewis, C., Williams, K., Herrick, C. J., Fields, M. E., Gray, T., Guo, Z., Nicol, G., and Baranski, T. (2024). “Effectiveness and Safety of Different Estradiol Regimens in Transgender Females: A Randomized Controlled Trial”. Journal of the Endocrine Society, 8(8): bvae108.
Doll, E., Gunsolus, I., Thorgerson, A., Tangpricha, V., Lamberton, N., and Sarvaideo, J. L. (2022). “Pharmacokinetics of Sublingual Versus Oral Estradiol in Transgender Women”. Endocrine Practice, 28(3): 237-242.