There is no direct conversion between SSRI (Selective Serotonin Reuptake Inhibitors) drugs. Most practitioners, when comparing antidepressant doses, will review comparative doses as well as recommended dosing guidelines.
Lexapro (escitalopram) comes in the following doses:
- 5 mg
- 10 mg
- 20 mg
Zoloft (sertraline) comes in the following doses:
- 25 mg
- 50 mg
- 100 mg
The maximum daily dose set for Lexapro (escitalopram) by the manufacturer is 20 mg per day. Doses above that (as per your question) are rare but have been used clinically in some situations. Comparing 40 mg of Lexapro (escitalopram) to Zoloft (sertraline) would put it at the top dosing range of Zoloft (sertraline), which is 200 mg per day (the maximum daily dose of the drug) or possibly more.
If you are switching from one antidepressant to the other, that is not uncommon. Over two-thirds of patients don't achieve remission on their first antidepressant medication. When switching between drugs in the same class (e.g. between SSRI drugs), there are some general recommendations but no clear guidelines and there is no "one size fits all" approach.
It is highly recommended to speak with your doctor regarding the best way to switch drugs. Your doctor may recommend either a direct switch between drugs or a method of tapering.
Stop the first agent and start a dose of the new agent that is in the same range as the first agent. In your case, it would be a high dose of sertraline. When switching between SSRI drugs in which you were on high doses, the direct switch method is not recommended due to an increased risk of adverse reactions and serotonin syndrome. It takes time to wash out the prior drug from you system and when you do a direct switch, you essentially will have high doses of both drugs in your system.
Starting out, gradually reduce your dosage of escitalopram to a maximum of 10 mg/day or lower. This taper could take as long as 4-6 weeks. After the taper period, switch to sertraline at 50-100 mg and taper up to effectiveness.
There are also methods where you can do a cross taper, meaning you taper down on one medication and gradually increase on the other.
The above are just general recommendations and for informational purposes. Every situation and patient is different. Be sure to discuss with your doctor the best possible options for you.