What Happens in a Phase 1 Clinical Trial?

If you are thinking about signing up for a Phase 1 clinical trial, you probably want to know what actually happens once you walk through the doors of a research clinic. The internet is full of vague descriptions and corporate FAQ pages, but not much that tells you what the experience is really like from the participant's side.

This guide walks through the entire process — from the first phone call to your last follow-up visit — based on how Phase 1 studies actually run at the contract research organizations (CROs) that conduct most of this work. Whether you are thinking about enrolling for the first time or just want to understand what you are getting into, this is the practical breakdown you need.

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Phase 1 is where every new drug starts in humans. You are among the first people on the planet to receive the compound, and that is exactly why the monitoring is so thorough.

What Phase 1 Actually Means

Phase 1 is the first stage of testing a new drug in human beings. Before it reaches you, the compound has been through years of laboratory and animal testing, but no one knows yet how it will behave in a living human body. That is the entire point of Phase 1: to find out.

These studies are sometimes called "first-in-human" (FIH) trials. They are not testing whether the drug works for a disease — that comes later in Phase 2 and Phase 3. Phase 1 is focused on three things:

• Safety and tolerability — Is the drug safe at various doses? What side effects does it cause?
• Pharmacokinetics (PK) — How does the body absorb, distribute, metabolize, and eliminate the drug? How long does it stay in your bloodstream?
• Dose ranging — What is the right dose to move forward with? Studies often start at very low doses and escalate gradually across different groups of volunteers.

Because the drug has never been given to humans before, Phase 1 studies almost always enroll healthy volunteers — people who do not have the disease the drug is meant to treat. This way, any side effects can be attributed to the drug itself rather than an underlying condition. It also means compensation tends to be high, because you are taking on the uncertainty of a novel compound. Learn more about compensation.

The Screening Process

Before you set foot in a research clinic for an actual study, you have to pass screening. This is a two-step process: a phone screen and an in-person visit. Not everyone makes it through, and that is by design — the researchers need participants who meet very specific health criteria.

Phone Screen

The phone screen is your first interaction with the clinic. A recruiter will ask basic qualifying questions: your age, weight, height, general health history, current medications, smoking status, drug use, and when your last clinical trial was (if ever). This call takes 10 to 20 minutes and is not compensated. If you meet the basic criteria, they will schedule you for an in-person screening visit.

Be honest during this call. Clinics check everything later with bloodwork and physical exams. If you lie about smoking, drug use, or medications, it will show up in your labs and you will be disqualified — and potentially flagged for future studies at that facility.

In-Person Screening Visit

The in-person screening visit is essentially a thorough physical examination. Plan for it to take 3 to 5 hours. Here is what typically happens:

• Informed consent — A staff member walks you through the entire study protocol, including the drug being tested, all procedures, potential risks, compensation, and your right to withdraw at any time. You sign the consent form. Read it carefully. Ask questions. This is not a formality.
• Medical history review — A detailed discussion of your health history, surgeries, family history, allergies, and medications.
• Physical examination — Vitals (blood pressure, heart rate, temperature), height, weight, and a head-to-toe physical exam by the study physician.
• Blood and urine tests — Multiple tubes of blood drawn for a complete metabolic panel, liver function, kidney function, blood counts, HIV, hepatitis B and C, and a drug screen. Urine sample for drug screen and sometimes pregnancy test.
• ECG (electrocardiogram) — Electrodes placed on your chest to check your heart rhythm. Takes about 5 minutes.
• Sometimes additional tests — Depending on the study drug, you might also get a chest X-ray, pulmonary function test, or more specialized labs.

After screening, the clinic reviews your results. If everything looks good, you will get a call within a few days to a week confirming your enrollment and giving you a check-in date. If something comes back abnormal — elevated liver enzymes, a positive drug screen, an ECG irregularity — you will be screen-failed. Most clinics pay $50 to $150 for completing the in-person screening visit, even if you do not qualify.

Check-In Day

Check-in day is when the study actually begins. You will arrive at the clinic in the afternoon or evening (typically between 3 PM and 7 PM, depending on the site). Here is what to expect:

You will go through another round of vitals, a brief physical exam, and baseline blood and urine tests. This is called the "Day -1" assessment — confirming that your health has not changed since screening. If you fail any of these checks, you can be sent home before the study starts and replaced with an alternate. That is rare, but it happens — especially if you show up with a positive drug screen or elevated vitals.

After medical checks, you will be assigned a bed, given hospital-style ID wristbands, and shown around the unit. Most Phase 1 clinics are set up like a cross between a hospital ward and a dormitory. You might have a semi-private area with a curtain or a shared room with 2 to 6 other participants. You will stash your belongings, change into whatever the clinic requires (some require hospital gowns during dosing, others let you wear your own clothes), and settle in.

Dinner is served on the clinic's schedule. Phones and laptops are almost always allowed, and most clinics have Wi-Fi. Then you wait. Check-in evening is usually uneventful — the real action starts on dosing day.

Daily Life During an Inpatient Stay

The biggest surprise for first-time participants is how much downtime there is. Between the scheduled procedures — blood draws, vitals, ECGs, meals — you are mostly just waiting. Here is what a typical non-dosing day looks like:

• 6:00 - 7:00 AM — Wake-up. Vitals check (blood pressure, heart rate, temperature). Fasting blood draw if scheduled.
• 7:30 - 8:00 AM — Breakfast. You eat what the clinic serves. Meals are standardized and controlled because food can affect drug metabolism. Do not expect gourmet — think institutional cafeteria. Some clinics are better than others.
• 8:00 AM - 12:00 PM — Free time with possible blood draws or ECGs scattered throughout. This is when most people read, watch shows, play video games, study, or work on laptops.
• 12:00 - 12:30 PM — Lunch. Same deal as breakfast — standardized, timed, and you have to finish within the allotted window.
• 12:30 - 5:00 PM — More free time, more occasional procedures.
• 5:30 - 6:00 PM — Dinner.
• Evening — Free time. Sometimes an evening vitals check. Snack if the protocol allows it.
• 10:00 - 11:00 PM — Lights out (or at least quiet hours). Some studies require you to be in bed at a specific time.

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The hardest part of a Phase 1 study is not the needles — it is the boredom. Bring something to do. Your future self will thank you.

A few things about daily life that nobody tells you upfront: You will eat on their schedule, not yours. Snacking between meals is usually prohibited. You cannot leave the unit — not even to walk around the building. Showers are available but may be scheduled. Your sleep will be interrupted if the protocol calls for nighttime blood draws. And you will get to know your fellow participants very well — for better or worse. Bring headphones.

Dosing Day

Dosing day is the most intense day of the study. This is when you actually receive the investigational drug (or placebo — most Phase 1 studies are randomized and blinded, so you will not know which one you got).

The morning starts early. You will be fasting (typically since the night before). Staff will take pre-dose vitals and draw pre-dose blood. An IV catheter may be placed in your forearm for PK blood sampling — this is a small plastic tube that stays in your vein so they can draw blood without sticking you with a new needle every time.

When it is time to dose, the process depends on the formulation:

• Oral tablets or capsules — You swallow the drug with a measured amount of water (usually exactly 240 mL). Staff will check your mouth to confirm you actually swallowed it.
• Oral solutions — You drink the drug solution. The container may be rinsed and you drink the rinse to ensure you received the full dose.
• IV infusion — The drug is administered through an IV line over a set period (30 minutes to several hours). You sit or lie still during the infusion.
• Subcutaneous injection — An injection under the skin, usually in the abdomen or thigh. Takes seconds.

After dosing, the monitoring ramps up significantly. For the first several hours, staff will check your vitals every 15 to 30 minutes. You will be asked how you feel frequently. ECGs will be taken at pre-defined time points. And the PK blood draws begin.

PK Blood Draws

Pharmacokinetic sampling is the backbone of Phase 1 research, and it is what makes dosing days exhausting. The study team needs to measure exactly how much drug is in your blood at precise time points to map out the drug's absorption and elimination curve.

On a heavy PK day, you might have blood drawn at these time points after dosing: 15 minutes, 30 minutes, 45 minutes, 1 hour, 1.5 hours, 2 hours, 3 hours, 4 hours, 6 hours, 8 hours, 10 hours, 12 hours, 16 hours, and 24 hours. That is 14 draws in a single day, and some studies add even more time points. It is not unusual to have 15 to 20 PK draws on dosing day.

Each draw takes only a minute or two (less if they are using a catheter), but the schedule is rigid. If the protocol says the 2-hour draw happens at exactly 2 hours post-dose, the nurse will be at your bedside at that exact minute. You cannot be in the shower, on a walk, or otherwise unavailable. On PK-intensive days, you are essentially tethered to your bed.

The total volume of blood drawn over an entire study can be significant — sometimes 500 mL or more across all draws. The clinic monitors your hemoglobin to make sure you are not becoming anemic. Your body replaces the blood naturally, but if you are doing back-to-back studies, this is one reason the washout period exists.

Common Side Effects and Monitoring

Because Phase 1 drugs are new, the side effect profile is not fully known yet. That is literally what the study is trying to figure out. However, based on preclinical data and the drug's mechanism, the investigators will have a general idea of what to watch for. Common side effects seen across Phase 1 studies include:

• Headache — The single most commonly reported side effect in Phase 1. Often mild.
• Nausea — Especially with oral medications taken on an empty stomach.
• Dizziness or lightheadedness — Particularly with drugs that affect blood pressure or the central nervous system.
• Fatigue — Feeling unusually tired or sluggish.
• Injection site reactions — Redness, swelling, or soreness at the site of subcutaneous or IV injections.
• GI symptoms — Diarrhea, constipation, or stomach discomfort.

Every side effect you experience — no matter how minor — is documented as an "adverse event" (AE). The nursing staff will ask you regularly how you feel, and you should always report anything unusual. This data is critically important: it helps determine the dose that moves forward into Phase 2 testing. For a deeper look at safety considerations, see our guide to clinical trial risks.

Monitoring in a Phase 1 unit is constant. Nursing staff are present around the clock. Vital signs are checked multiple times per day. ECGs are performed at scheduled time points. Lab work is drawn regularly to check liver function, kidney function, and blood counts. If the study drug raises any safety signals, the principal investigator reviews the data and can pause or stop the study.

Discharge and Follow-Up Visits

Discharge day is what everyone looks forward to. After your final scheduled procedures (usually a last set of vitals, an ECG, and blood work), the study physician will do a brief physical exam to confirm you are safe to leave. If everything checks out, you sign discharge paperwork, collect your belongings, and walk out.

But the study is not over yet. Almost every Phase 1 trial includes one or more outpatient follow-up visits. These typically happen 5 to 14 days after discharge. During a follow-up visit, you will have blood drawn, vitals checked, a brief exam, and be asked about any ongoing symptoms. Follow-up visits usually take 1 to 2 hours and are compensated separately.

For multi-period studies (like crossover designs where you receive different doses or formulations in each period), you will be discharged after Period 1, have a washout period of 7 to 14 days at home, and then check back in for Period 2. Some studies have three or even four periods. You are typically paid after completing each period, with the final balance (and often a completion bonus) paid after the last follow-up visit.

The Washout Period Between Studies

After completing a Phase 1 study, you cannot immediately jump into another one. Every research clinic enforces a washout period — a mandatory gap between studies to ensure the previous drug has fully left your system and your body has returned to baseline. The standard washout is 30 days from your last dose of study drug, but some studies require 60 or 90 days, especially for drugs with long half-lives or those that might affect your immune system.

Clinics track this through shared databases. In the US, the most widely used is Verified Clinical Trials (VCT), which logs your participation dates and study drug information. When you screen for a new study, the clinic will check VCT to verify your last participation date. Trying to game the system by enrolling at two clinics simultaneously is dangerous (drug interactions are unpredictable) and will get you permanently banned from research.

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The washout period is not optional and it is not bureaucratic overhead. It is a safety mechanism that protects you, the study data, and future participants.

How to Prepare for Your First Phase 1 Study

Preparation is the difference between a smooth experience and a miserable one. Here is what to bring and what to know before you check in:

What to bring:

• Laptop, tablet, or e-reader — you will have hours of downtime every day
• Phone charger and a long charging cable (outlets may not be right next to your bed)
• Headphones or earbuds (essential for shared rooms)
• Comfortable clothes — sweatpants, t-shirts, slippers or slides
• Toiletries and any personal hygiene items
• Books, puzzle books, playing cards, or a portable gaming device
• Schoolwork or work materials if you need to stay productive
• A light blanket or pillow if you are particular about sleep

What to know:

• Read the informed consent document completely. Know the drug name, what it does, the known risks, and the compensation schedule.
• Arrange your life for the confinement period — bills, pets, mail, work coverage. You cannot leave once you check in.
• Avoid alcohol for at least 48 hours (ideally longer) before check-in.
• Do not take any medications or supplements without telling the study team — even over-the-counter pain relievers can disqualify you.
• Eat clean in the week before the study. Some diets or foods can affect liver enzymes and other lab values.
• Get good sleep before check-in. Sleep can be disrupted during the study, so bank rest beforehand.

Phase 1 studies are a legitimate way to contribute to medical research and earn meaningful compensation while doing it. They are not for everyone — the confinement, the blood draws, the dietary restrictions, and the inherent uncertainty of a new drug are real trade-offs. But for people who are comfortable with those trade-offs, it can be a surprisingly manageable experience. The monitoring is thorough, the staff are professional, and the vast majority of participants complete their studies without any significant issues.

Ready to look at what is available? Browse Phase 1 pharma studies or search all clinical trials.

Frequently Asked Questions