Day 1 of a Clinical Trial: What Happens When the Study Begins

The drive to the research site on the morning of Day 1 feels different from the earlier visits. By this point, you have already met with the study team, signed the informed consent form (the document that explains the study and your rights as a participant), and completed a screening visit (the visit that confirms you meet the study’s eligibility criteria). Today is the first day you actually take part in the study.

In clinical research, “Day 1” almost always refers to the first day a participant receives the investigational product, often called the study drug. It is not the same as your screening visit, and it is not the same as the day you signed your consent form. It is the day the study clock formally begins for you, and the way it unfolds is shaped by the study’s protocol (the written plan that governs every step of the trial).

What “Day 1” Actually Means in a Clinical Trial

Clinical trials use specific vocabulary for visits because the order matters for safety and data quality. Day 1 sits after two earlier milestones: informed consent and screening. Once you have passed both, Day 1 is the day the first dose is given and the first set of on-study measurements is taken.

If you are looking for a more general overview of what your earliest study visit involves, the broader topic is covered in What to Expect at Your First Study Visit. This blog stays focused on the dosing day, because Day 1 has its own rhythm that is different from a baseline visit without dosing.

The structure of Day 1 depends on what kind of study you are in. An oral pill given once a day in an outpatient cardiovascular study looks very different from an infusion given over several hours in a hospital research unit. What stays consistent across studies is the order of events: arrival, identity and eligibility confirmation, pre-dose checks, dose administration, post-dose monitoring, and discharge instructions.

Before You Arrive: Preparing the Night Before and Morning Of

The instructions for the night before Day 1 come directly from the study protocol and from the research coordinator who scheduled your visit. These instructions are not generic. They are tailored to the study drug and to the assessments planned for that day.

Common Day 1 preparation instructions include:

• Fasting for a specified number of hours before the visit if blood samples are part of the day’s procedures
• Holding (skipping) certain regular medications for a defined window before the visit, only when the study team has told you to do so
• Avoiding alcohol, caffeine, or specific foods that could interfere with the study’s measurements
• Drinking water normally unless told otherwise
• Bringing a list of all current medications, including over-the-counter products and supplements, for the team to review

On the morning of Day 1, you will usually be asked to arrive earlier than a routine clinic appointment. Studies often build in time for re-checking eligibility, drawing baseline labs, and walking through the day’s plan before any dose is given. Bring your government-issued photo ID, your insurance card if requested, your medication list, and any items the coordinator told you to bring (a journal, a wearable device, or paperwork). A more complete view of what is typically required before any enrollment step is in Pre-Study Requirements: What You Need Before Enrolling.

Check-In and Pre-Dose Procedures

After you check in at the research site, the first thing the study team does is confirm your identity and verify that nothing has changed since your screening visit. This step is not a formality. New medications, recent illnesses, or changes in your health since screening can affect whether the study can safely proceed with dosing on that day.

Pre-dose procedures typically include:

• A vital signs check (blood pressure, heart rate, temperature, and sometimes oxygen levels)
• A brief physical exam, often focused on the body system the study is investigating
• Blood draws for baseline lab values that the team will compare against later samples
• A urine sample, if required by the protocol
• A pregnancy test, if applicable, because many studies cannot dose during pregnancy
• An ECG (an electrocardiogram, a recording of your heart’s electrical activity) for studies involving drugs that can affect the heart
• A review of any new symptoms or medication changes since your last visit

This is often the longest part of Day 1. Some participants are surprised by how thorough the pre-dose work is, but everything that happens before the first dose is part of the study’s safety system. If a baseline value is unexpectedly out of range, the team may pause dosing or contact the study’s medical monitor before proceeding. If you have questions about what each step is for, this is a good time to ask. Many participants find it helpful to bring questions written down in advance, and a useful starting list is in Top Questions to Ask Before Joining a Clinical Study.

The First Dose: What Actually Happens

Once the team has cleared you for dosing, the principal investigator (the doctor or qualified researcher who leads the study at that site) or a designated study clinician administers the first dose. The route depends entirely on the study. Some examples of how a first dose is given:

• An oral capsule or tablet, taken with a measured volume of water under direct observation
• A subcutaneous injection (an injection just under the skin), often in the arm or abdomen
• An intramuscular injection (an injection into a muscle)
• An intravenous (IV) infusion, given through a small catheter placed in a vein, sometimes lasts from 30 minutes to several hours
• An inhaled medication, delivered through a specific device, was studied

If the study is blinded (a setup in which neither you nor the study team knows whether you are receiving the study drug or a comparator), the dose may look identical to a placebo (an inactive substance used for comparison) or to a different dose level. The reasons studies use these designs are explained in Placebos and Controls: What It Means in Your Study, which is worth reading before Day 1 if you have questions about what you might be receiving.

The team records the exact time of the first dose. This time becomes the reference point for every subsequent measurement on Day 1 and, in some studies, for the entire dosing period.

After the First Dose: Monitoring, Discharge, and the Days That Follow

Post-dose monitoring is the most variable part of Day 1. Some studies require you to stay at the research site for several hours of structured observation, with vital signs, blood draws, and symptom checks at scheduled intervals. Other studies discharge you within a short window after dosing, with instructions to call the study team if anything unexpected happens.

Specific post-dose activities can include:

• Repeated vital sign checks at fixed time points (for example, at 30 minutes, 1 hour, 2 hours, and 4 hours after dosing)
• Pharmacokinetic blood samples, which measure how the study drug is absorbed and cleared by your body
• A symptom check by the study nurse or coordinator at each time point
• Observation for any immediate adverse events (any unwanted medical occurrence, whether or not the team believes it was caused by the study drug)
• Confirmation that you are stable enough to leave, when applicable

Before you leave, the team will go over what to expect over the coming hours and days, what symptoms to watch for, when to call, and which symptoms warrant going to an emergency department instead of waiting. You will usually be given a 24-hour contact number, a copy of the day’s instructions, and any take-home study materials, such as a diary, a wearable device, or the next dose to take at home if the protocol allows. Following these instructions carefully matters for both your safety and the quality of the study’s data, and the broader expectations for participants during a study are covered in Staying Healthy and Compliant During a Trial.

How to Find a Clinical Trial That Might Fit You

Every Day 1 starts with a study that the participant has already chosen, consented to, and screened into. Finding the right study earlier in that journey is what makes Day 1 less unfamiliar.

DecenTrialz is a platform that connects people to clinical trials they may be eligible for. You share some basic information about yourself, get matched with studies that could fit, and talk to a nurse who walks you through what each study involves before you decide whether to move forward. The research team running the study makes all final eligibility and enrollment decisions. You can start a search at decentrialz.com.