Starting an Antidepressant: What to Expect in the First Weeks

You just filled your first antidepressant prescription. Maybe you feel hopeful, maybe nervous, maybe both. Knowing what usually happens in the first few weeks can make this time easier. This guide covers the timeline, common side effects, warning signs, and what your clinician watches for.

How antidepressants work

Antidepressants are medications that change how certain brain chemical systems, such as serotonin and norepinephrine, send signals [2]. The most commonly prescribed types are:

• SSRIs (selective serotonin reuptake inhibitors), such as sertraline, escitalopram, citalopram, and fluoxetine
• SNRIs (serotonin and norepinephrine reuptake inhibitors), such as venlafaxine and duloxetine
• Other types, such as bupropion, mirtazapine, and vortioxetine

A large review of 522 trials found that all 21 antidepressants studied worked better than placebo for adult depression [5]. No single drug is best for everyone. Clinicians choose based on side effects, other health conditions, other medications, past response, and cost.

The usual timeline

Week 1 to 2

• Side effects often show up first, before you feel any mood benefit.
• Some people notice better sleep or appetite early, especially with mirtazapine.
• Energy may improve before mood does.

Week 2 to 4

• Many people notice the first signs of improvement during this window [1].
• Early side effects such as nausea and headache often ease.
• Your clinician may check in to see how you are doing and whether the dose is right.

Week 6 to 8 and beyond

• Full benefit often takes 6 to 8 weeks, and sometimes longer [2].
• The goal is remission, meaning your symptoms are mostly gone, not just a little better.

Common side effects

Side effects depend on the medication. For SSRIs and SNRIs, common ones include [3]:

• Nausea, upset stomach, or diarrhea
• Headache
• Trouble sleeping, or feeling sleepy
• Feeling jittery or restless
• Sweating
• Sexual side effects, such as lower desire or trouble reaching orgasm

Many of these fade within 1 to 2 weeks. Sexual side effects are an exception and can persist. Tell your clinician about them, since there are often ways to help, such as changing the dose or switching medication. Mirtazapine more often causes sleepiness and weight gain. Bupropion is less likely to cause sexual side effects but can cause trouble sleeping.

Tips people often find useful

• Ask whether to take the dose in the morning or at night.
• Taking it with food may ease nausea if your pharmacist says that is fine.
• Keep a short daily note of mood, sleep, and side effects to share at follow-up.

What your doctor looks at

Clinicians follow guideline and drug label safety points when you start an antidepressant.

Follow-up timing

• CANMAT guidelines recommend checking side effects early and assessing improvement at about 2 to 4 weeks [1].
• If there is little or no improvement after 2 to 4 weeks at a good dose, CANMAT suggests reviewing the plan, such as raising the dose or switching [1].
• NICE (UK) guidance recommends a review within 2 weeks of starting, and within 1 week for people under 25 or at higher risk of suicide [4].

Suicidal thoughts in younger people

The FDA requires a boxed warning (its strongest warning) on all antidepressants. It says that in children, teens, and young adults under 25, antidepressants can increase suicidal thoughts and behaviours, especially in the first months and after dose changes [2]. Health Canada product monographs also warn about agitation-type changes and self-harm thoughts and advise close monitoring early in treatment. For most adults, treating depression lowers suicide risk overall, but everyone should watch for sudden changes.

Other safety checks

• Serotonin syndrome: a rare but serious reaction, more likely when combining medications that raise serotonin, such as some migraine and pain drugs, St. John's wort, or a second antidepressant [3].
• Bleeding risk: SSRIs can slightly raise bleeding risk, especially with blood thinners, aspirin, or NSAIDs such as ibuprofen.
• Low sodium (hyponatremia): more likely in older adults, and can cause confusion or unsteadiness.
• Mood switching: in people with bipolar disorder, an antidepressant alone can trigger mania. Your clinician may ask about past periods of very high energy and little need for sleep.
• Pregnancy and breastfeeding: tell your clinician if you are pregnant or planning to be.

Canada and US differences

Both the FDA label and Health Canada monographs warn about suicidality, serotonin syndrome, and stopping suddenly. The FDA boxed warning names the under-25 age group. Health Canada warnings apply to patients of all ages. Brand names and covered drugs can also differ between countries and insurance plans.

Do not stop on your own

Stopping an antidepressant suddenly can cause discontinuation symptoms, such as dizziness, nausea, and electric-shock feelings. If side effects are a problem or you want to stop, talk to your clinician or pharmacist first so you can plan a safe change [4].

Questions to ask your clinician

• Why did you choose this medication for me?
• Which side effects are most common, and which ones should I call about?
• When should I expect to notice a difference?
• When is my follow-up visit, and who do I contact before then?
• Does this medication interact with anything else I take, including supplements?
• What should I do if I miss a dose?
• Should I also start therapy at the same time?

When to get care quickly

Get help right away if you notice:

• New or worse thoughts of suicide or self-harm
• Severe agitation, panic, or feeling unlike yourself
• Very high energy, racing thoughts, and little need for sleep
• Fever, heavy sweating, fast heartbeat, muscle stiffness or twitching, and confusion together, which can be signs of serotonin syndrome
• Signs of an allergic reaction, such as swelling of the face or trouble breathing

In Canada and the US, call or text 988 for suicide crisis support. Call 911 or go to the nearest emergency department if you are in immediate danger.

Bottom line

The first weeks on an antidepressant are often the hardest. Side effects usually come first and often fade, while mood benefits build over 2 to 8 weeks. Keep your follow-up visits, track how you feel, and raise concerns early. Never stop or change your dose without talking to your clinician or pharmacist.

Primary AI can show you the guideline and study sources behind this topic in plain language, so you can bring better questions to your appointment.