There is no feminine form of médecin to speak of. It is used both for men and women, even though it is a masculine noun. If necessary, one could say “une femme médecin“.

The Office québécois de la langue française, the Quebec French regulation office, which has been very pro-feminization of profession titles in the 1980’s-1990’s, says the following about médecin in its job title feminization guide (dated 1991):

Les féminins réguliers de marin et médecin seraient, respectivement, marine et médecine, formes déjà utilisées pour désigner ces domaines d’activité. L’homonymie pouvant être gênante, l’Office propose de considérer marin et médecin comme épicènes. On aura donc : une marin, une médecin.

What it is saying is that adding an E to médecin creates an already existing word and that the newly created homonym would be awkward. So the proposal is to consider médecin as epicene, and that is how it is used today.

“Doctoresse” is the French feminine form of doctor. http://fr.wiktionary.org/wiki/doctoresse

“Doctoresse” est le feminin de docteur. http://fr.wiktionary.org/wiki/doctoresse

There is no feminine for médecin, it is a noun of masculine gender which can refer to person of both sex (just like personne is a noun of feminine gender without any hint about the sex of the person).

There are some tentatives from official committees to give a feminine to jobs title, Kareen gave the Quebec one, here is the French one. I don’t know the situation in Quebec but as far as I can tell, here the recommendations have not influenced the usage, and une médecin or une médecine aren’t used (they aren’t even cited in dictionaries I’ve consulted).

For docteur the situation is nearly similar, i.e. un docteur doesn’t offer any indication about the sex. But une doctoresse exist (and is restricted in usage to practitioners of medecine) and I’ve seen and heard une docteur and une docteure (for any kind of docteur) even if dictionaries ignore them.

There was a big discussion over gender equality and job titles.

It all began in 1984 when the prime minister (Pierre Mauroy, recently deceased) asked the general comission for terminology and neology about feminisation of job titles. The commission answer was clear, they were oposed to the project.

Despite the negative answer and due to political pressure, the french documentation published a catalogue of feminised name.

The french Academy had to intervene. Professors Georges Dumezil and Claude Levi-strauss write the text and give their position. The better way to fight gender inequality is to consider jobs titles as collective and neutral. this would also avoid language “heaviness”. A reinforcement of the female position in the society by a feminisation of jobs title would be an illusion. Furthermore, a differentiation could be seggregating in some case, so the effect could be in total opposition to the initial goal of the feminisation.

The french academy advised not to use the feminised job titles.

They doesn’t reccomend it but if the differentiation is really needed, they advised to use “la” before the job title. The official answer to your question is “la docteur”.
The only exception would be for military grade and honorific titles (“docteur” is one of these titles), never ever use “la”.

On the other hand, feminists argued that this wasn’t a grammar question but a sociolinguistical one. That job titles are considered masculine in everyone’s mind and so can build resistance to feminisation of the workplace.

Note that in france, thanks to the academy, the “neutralists” won over the “feminists” and it is always akward to use a feminised name. La docteur could be a “neutralist” or a “feminist” and the better ways to go are to avoid using it or asking her choice.
From what I’ve heard, it is not the same in Canada, where the list of feminised name have been accepted. In Canada you can say “la doctoresse”.

See here for the text: http://www.academie-francaise.fr/actualites/feminisation-des-noms-de-metiers-fonctions-grades-et-titres

Feminisation of the language is a false problem… it looks like that the rights we want to install in our societies need to have aftereffects with the grammar of the language to exist : this is non-sense and a pain for the ear.

In France, the Académie française is the guarantor of the language against barbarisms and other insults.

Read this article, which was already suggested by lmorin, and you will understand everything.

The Academy has recognised the feminisation of names and you’ll likely see docteure more often than doctoresse. Here is an article on the subject: https://www.lemonde.fr/societe/article/2019/02/28/l-academie-francaise-se-resout-a-la-feminisation-des-noms-de-metiers_5429632_3224.html